Experienced and professional animal trainer provide their insights in answering this question :
A. It sounds unspecific. You should see a vet to check what is going on. It can be connected with breathing difficulties, neurological problem, pain etc

How to Identify Common Pet Problems ?

Our sources include academic articles, blog posts, and personal essays from experienced pet care professionals :

When your dog is relaxed and happy, he`s likely to have his mouth closed or slightly opened. If his mouth is open, he may be panting—this is how dogs cool their bodies. You might see his teeth because his mouth is slightly opened. A dog who`s frightened or feeling submissive probably has his mouth closed.
Dogs regulate their body temperature through their mouths and after exertion or when the weather is hot your dog will pant to cool down his body temperature. Your dog also uses an open mouth expression to show he is relaxed and happy.
Most dogs mouth you because they`re simply too excited about something. In some cases, they may be aroused by a treat in your hand – in which case you work on teaching your dog to take treats gently. Other times they may be amped up due to play or some other stimulus.
A canine who has lockjaw has the inability to completely open or close his mouth. Illness, arthritis, tetanus and neurological disorders are just a few reasons why a dog may have this condition.
However, dogs can make their faces appear to smile. This happens when they open their mouths and pull back their lips, letting their tongues lap over their teeth. This “dog smile” usually occurs in situations when they are relaxed and appear happy, which is why dog owners think it is a smile.
A relaxed dog will likely have his mouth open and may be panting, with no facial or mouth tension. The corners of his mouth may be turned upward slightly. A fearful or tense dog will generally keep his mouth closed, and may pull his lips back at the corners (also known as a “long lip”.) He may also be panting rapidly.
Some dogs, however, are either partially or fully unable to control their tongue and it may hang out of their mouth on a consistent basis. These dogs have a condition know as hanging tongue syndrome, and it can be triggered by a congenital defect, injury, dental disease or neurological damage.
The other way is by panting, which is when the dog will breathe in and out quickly with their tongue out. By doing this they rapidly take cooler air into the body to bring their temperature down. Drawing air over the wet tongue also has a similar cooling effect to sweat on human skin.
Signs Your Dog is Experiencing Oral Pain

Drooling. Whimpering. Halitosis (bad breath) Change in behavior or displaying protective behavior.

Many dogs will put their mouth and paw on us – not with any intention to hurt, but more to attract play and affection. This is known as mouthing and pawing. This most commonly occurs with puppies, but often continues into adulthood.
In the case of a stroke, your pet`s brain sends incorrect signals to their body. For example, your pet may not be able to respond to your spoken directions. Instead, he may move in a directionless fashion and seem to be walking in circles. He may appear as though he is drunk and be unable to walk in a straight line.
Dysphagia in Dogs. Dysphagia, the medical term given to difficulty swallowing, can occur anatomically as oral dysphagia (in the mouth), pharyngeal dysphagia (in the pharynx itself), or cricopharyngeal dysphagia (at the far end of the pharynx entering the esophagus).
Studies show that smiling at your dog can boost his or her levels of oxytocin, also called “the love hormone” for its role in building attachment and trust. On top of that, new research suggests that seeing a human smile might even cause dogs to ignore their natural instincts in favor of our puppy love—pun intended.
A pup can definitely pant and feel happy too. But to know for sure, you need to assess the big picture. Your dog`s smile could also be a nervous grin or a sign that your pup is stressed, uncomfortable, or sending a warning. Therefore, when your pup appears to be sending a smile your way, assess their body language.
Confidence in Dogs

He holds himself upright yet relaxed, with his head up and a bright-eyed, alert expression. His tail may be raised slightly, and his ears may be pricked up if something has gained his interest, or relaxed and floppy if he`s just hanging out in your company or the company of other dogs.

We humans tend to get warm and fuzzy feelings when we see dogs “smile.” It`s true that some dogs` mouths open in a cute smile when they are relaxed and happy. But a dog with his mouth open could alternatively be panting from pain, stress, or fear.
Dogs change their facial expressions when they know people are looking at them—perhaps in an effort to communicate. For instance, canines in the study would make the classic “sad puppy face”—raising their inner eyebrows to make their eyes look larger and more infant-like—when looking into a human`s eyes.
Licking is a natural and instinctive behaviour to dogs. For them it`s a way of grooming, bonding, and expressing themselves. Your dog may lick you to say they love you, to get your attention, to help soothe themselves if they`re stressed, to show empathy or because you taste good to them!
Dogs will lick their lips non-stop as an appeasement gesture when feeling threatened, anxious, or nervous or when they experience a health-related problem such as nausea, oral discomfort, allergies, or internal pain.
Wincing, Whining or Crying

All three of these actions indicate an injury or some kind of pain your dog is experiencing. If you start to pet your dog and they shy away from your hand or whine, you know there is something wrong.

Your dog might jump on you, lick your face, and they`ll definitely wag their tail. Being excited and happy to see you is one way you can be assured they love and miss you. They seek physical contact. This can come in the form of a quick nuzzle, a cuddle, or the famous lean.
“Mouthing is when a dog puts his teeth and mouth over a person`s skin while using little or no pressure from his jaw.” says dog trainer and expert Mikkel Becker on Vetstreet.com. “It`s not to be mistaken for aggressive biting, which is done out of fear or frustration.”
He`s play-biting

Play-biting is a different thing altogether, and it can be adorable and a clear sign your dog wants some affection—pronto. “If your pet is play-biting (or pulling at you for attention), then he`ll do it because he`s having fun with you, and it`s a sign of affection,” explains Dr. Nelson.

Relevant Questions and Answers :

the most relevant questions and answers related to your specific issue

Q. I have a 13 1/2 year old Shih Tzu. How old is he in dog years?
ANSWER : A. It’s used to be that dog years were 7 years to every 1. Now it normally around 5 years to every year as long as your dog is healthy and kept up with vaccines. So he’s about 68ish in dog years.

Read Full Q/A … : Shih Tzu Age

Q. My 1 year old great Pyrenees will not walk on the leash she stops and jumps around on the leash and when I try to walk her she just sits down.
ANSWER : A. At one year, she is right in the middle of canine adolescence which is when new fears can show up. She needs positive reinforcement and luring to teach her leash walking is a good thing. You can take some special, tasty treats that she doesn’t get regularly for leash walking time only. Use these treats to lure her forward a little bit at a time. Don’t pull or force her or it will become a big, negative experience to only reinforce her discomfort. Keep it light, positive, and fun. You will likely only get a short distance the first few days, but within a few weeks you should be able to increase that and help her overcome her fear about it.

Q. Does an indoor cat need to be vaccinated every year?
ANSWER : A. In practice, I recommend a feline combo vaccine every year, but will generally start administering every 3 years once they have had their kitten vaccines and 2 additional yearly vaccines. Rabies, is required yearly by law, and if kept up to date can be good for up to three years also. Based on the age of your cat I would give a yearly feline combo and rabies, and then boost the combo again next year.

Q. What can I do to help our Lab stop pulling on the leash when we go for a walk? I have tried different techniques but after 2 1/2 years she still pulls
ANSWER : A. POST TWO:
In the mean-time, while you’re working on building up that attention indoors, you should be using a front hooking harness outdoors on your walks. This will eliminate your girls pulling power. The Sensible http://www.softouchconcepts.com/index.php/product-53/sense-ible-harness and the Sensation http://www.softouchconcepts.com/index.php/product-53/sense-ation-harness harness are the best front hooking harnesses on the market because they do not have the martingale loop on the front of the harness (which can cause the dog to yo-yo during walks).

Lastly, I’d just like to add that dogs sniff the ground during walks for added mental stimulation. If your dog isn’t allowed to sniff the ground, the walk isn’t nearly as fun or tiring. When you are practicing attention on your walks, make sure it’s in short, small bursts. Attention for a few steps, back to sniffing for several steps, attention for a few steps, sniffing for a few minutes.. etc.

Q. My 8 year old rottweiler has been walking around with his mouth open..why?
ANSWER : A. It sounds unspecific. You should see a vet to check what is going on. It can be connected with breathing difficulties, neurological problem, pain etc

Read Full Q/A … : Leerburg

Q. How can I keep my 14 year old Yorkie from snapping at the younger ones?
ANSWER : A. It’s all about management. Do not allow the 7yo’s to interact with your 14yo unsupervised. You should be there each time they interact so you can redirect the 14yo’s attention onto some toys, or onto some treats when the 7yo’s are around. It sounds like you need to help your 14yo make positive associations with being around the younger pups. You should be trying to feed him treats each time he interacts with them, and doesn’t snap at them. Pet and praise him each time he is around them, or any time they are near. As I said, keep the separated when you cannot supervise their interactions because if you aren’t around when he is snapping at them, you could end up with a fight on your hands.

It could also be that they spend too much time together. Imagine spending 100% of your time with somebody, day in and out, doing everything together… including going to the bathroom.. that might bother anybody. I think you should give them more time apart from each other. Take them all on separate walks, separate them to play with them individually, separate them when you take them to potty, separate feeding times in separate rooms, etc. This can help alleviate the stress your older dog is feeling due to living closely with other dogs. You should always be giving individual activities in a houseful of dogs anyway.. when you expect them to get along 100% of the time, that’s when you find trouble.

Q. I was wondering what would be the best dog for a nine-year-old 14-year-old and a nine-year-old that cannot walk
ANSWER : A. You should pick a gentle, no agressive, inteligent dog. some breeds are predisposed to this kind of character but you should always asses the single animal and its behaviour. You can choose from family breeds like labs, shephards, retrievers.

Read Full Q/A … : Caring for an Elderly Dog

Q. How do I teach my dog to sit still enough and not move his head while I clip on the gentle leader?
ANSWER : A. Most dogs HATE the gentle leader, and it’s not at all surprising. Would you want something foreign on your face? It’s an uncomfortable training tool, and no dog enjoys wearing it. If you are looking to have your dog behave better on-leash, you should consider tossing out that gentle leader, and using a front hooking harness like the Sensible http://www.softouchconcepts.com/index.php/product-53/sense-ible-harness, or the Sensation http://www.softouchconcepts.com/index.php/product-53/sense-ation-harness harness. These harnesses will eliminate the pulling power of your dog, and put you in control in a positive, and gentle way. Any time your dog pulls, he is redirected until he is facing you. You can practically walk your dog with your pinky.

I dislike the gentle leader because it can cause neck injuries in an avid puller/lunger. You also can’t ever hook a long-lead to the gentle leader and allow your dog to run around because it would break his neck. Another thing I dislike about it, is it discourages sniffing the ground during walks. When your dog attempts to sniff, and the leash is short, his nose is redirected upwards. When you trip on the leash, the head is jerked around and the nose is directed upwards. Sniffing during walks is extremely important. Sniffing = mental stimulation, which will tire your dog out more during your walks. The more your dog lags, or forges, the less he can sniff the ground, and the more frustrated he becomes.

If you’re dead set on using the head halti.. you should be using treats to hold his attention. Place the head halti on the floor, reward him for sniffing it, pick it up, treat him, put it near his face, treat him, lure his nose through the loop, lots of treats, take the head halti off, more treats, lure his nose through again, more treats. Take baby steps going forwards AND backwards so the “game” of getting the halti on isn’t always getting more difficult.