This is an interesting article about the science behind smiling, and most importantly the beneficial effects of smiling:
Smiling stimulates our brain’s reward mechanisms in a way that even chocolate, a well-regarded pleasure-inducer, cannot match.
The article even includes directions on how to practice your smile. That sounds off-putting initially (to me, at least), but it’s interesting that it’s specifically teaching you to practice your genuine smile. Fake smiles, apparently, just don’t give the same benefit.