Bare your teeth in happiness ?

I think some research has indicated that smiling has a psychotheraputic effect - the action stimulates the production of certain chemicals, which are generally beneficial.

Some other research suggest that smiles evolved from the expression of fear. If you think about it, that isn't such a bad idea, since fear responses are generally a submissive gesture, and so encourage social cooperation as smiles generally do today.

 

It's also worth noting that for humans, teeth are not what we fight with. Raising your hands is much more agressive than anything you can do with your face, especially if you're holding a stick.

A secondary issue is that smiling has more to do with the shape of the lips than it does with showing the teeth.

 

Dogs also smile, bearing teeth, when happy. Perhaps because predatory animals are happy when hunting and killing, using their chompers? We humans are, after all, predators.