Aggressive behaviors in dogs include biting, growling, curling the lip, and other threatening behaviors. Aggressive tendencies are rare in pugs, but any dog can become aggressive given the right circumstances or a poor temperament inherited from parents. Aggression is one of the many ways in which dogs communicate, but that doesn't mean it's okay for your pug to ever behave aggressively. Because he lives in a human family, he needs to learn to temper his behavior to human standards.
You can tell your pug is smiling by studying his body language. A smile is expressed with the lips pulled to the side and a tail that's wagging. This is typical friendly pug demeanor.
Dogs can show aggression toward their owners, toward strangers, or toward other animals. Although it doesn't seem that way to us, most forms of aggression are motivated by fear. Some fears that can cause aggression in dogs include an invasion of territory by a stranger, another dog, or a new baby in the home; the fear of a mother dog that her pups will be harmed by an approaching stranger; or the fear of being hurt physically in some way.
How to Prevent Aggression
You can prevent fear and possessive aggression with plenty of socialization in puppyhood. Puppy kindergarten, obedience class, and play dates at parks with other dogs and people are also good ways to deter aggression. Training helps your pug learn to defer to you as the family leader and protector of territory. Neutering at adolescence can also help reduce the incidence of aggression.
How to Deal with Aggression
If your pug behaves aggressively for no apparent reason, take him to the veterinarian to rule out a physical problem that could be causing pain. If that's not the issue, seek the help of a qualified behaviorist who can advise you on behavior modification techniques.