Everyone smiles, but for slightly different reasons. North Americans often smile to appear polite, which makes the rest of the world think we're weird (cause forced smiles never reach the eyes).

Spontaneous smiles activate the major cheek muscles (pulling the lips up), as well as the muscles just around the eye. Forced smiles only activate the cheek muscles (it's apparently nearly impossible to consciously activate those eye muscles).

There's also questions of degrees of smiling, as in amount of teeth.