When a dog bares his teeth he is using body language to communicate. It's important to have some understanding of dog body language so you can understand what your dog is trying to tell you.
What It Is
A dog bares his teeth by curling his lips back to reveal his teeth. It is a reflexive action that occurs in reaction to certain situations.
What It Means
Most of the time, when a dog bares his teeth he is sending you a clear message to back off. It is a precursor to more serious aggression. This type of dog body language is often accompanied by vocalizations, such as snarls and growls. You may also notice other body language that indicates a dog is becoming aggressive, such as erect ears, a rigid body posture, and a tail that is held high and moving back and forth rapidly.
Dogs become aggressive for a number of reasons - resource guarding, territoriality, fear. No matter the reason a dog becomes aggressive, bared teeth are a sign he is being pushed past his comfort zone. If your dog is baring his teeth at you, you should seek the help of a dog trainer or behaviorist to help manage or put an end to the problem.
Occasionally, a dog bares his teeth without any aggressive tendencies behind it. This is referred to as a submissive smile. It is usually accompanied by body language such as lip licking, an averted gaze, a relaxed body posture, and ears sitting flatter against the head. Not all dogs exhibit submissive smiles.
If you are unsure whether your dog is exhibiting a submissive smile or becoming aggressive when he bares his teeth, your best bet is to call in the help of a professional dog trainer or behaviorist.