Ask an expert on dog behavior why dogs smile, and you may get a somewhat complicated answer involving submissive behavior, automatic responses, etc. They often ruefully conclude, "But dogs don't really smile, at least, not for the same reasons we do."
Ask the average dog owner the same question, and he'll respond indignantly: "Of course he smiles! Just look at that face; he's smiling right now – you can't say that's not a smile!"
We'll look at both sides of this question, because, in a way, both answers are correct.
In wolves, the expression we take for a smile indicates nervousness or submission to another wolf. Dogs also are hard-wired to interpret the expression this way. To signal that he accepts his subordinate position, a subordinate dog retracts the corner of his lips, which pulls the mouth into that happy face we recognize as a smile.
In reality, the "smile" is indicating that the dog is not threatening to overturn the status quo.
Confident alpha dogs rarely smile. They think of themselves as our equals and have no need or desire to signal their deference to us. Consider two men in a bar, one smaller than the other. The smaller man smiles to placate the bigger guy and avoid a conflict
An exception is the "smile" that comes with teeth bared – a smile that decidedly does not indicate submissiveness. A dog that lifts his lip to show his fangs is giving a signal to an encroacher to back off ... NOW.
But dogs are, if nothing else, extremely astute observers. An owner sees his dog "smiling" at him without fangs showing. The owner, touched by this adorable face, rewards the dog with a belly rub, a pat on the head, treats, or some other form of positive attention. The dog notices that when he performs this submissive gesture, he gets something desirable in return. So in a way, the dog's "smile" may become genuine, because he associates the expression with something that makes him happy.
This also helps explain why dogs roll on their backs for those treasured belly rubs. Rolling onto the back normally indicates submission, which the dog is happy to concede because of all the perks that come with it. For the dog, acting submissively is a pretty good deal.
