Is it strange that when we're pleased, the corners of our mouths turn up? Smile reflexes can be seen in infant babies as young as two months old...so...
Why should we smile?
Smiling reduces stress. While both authentic and 'forced' smiles work in this regard, the effects of a genuine smile are more prominent than the fake one - but only slightly - so don't be afraid to wear a smile if you're under stress - it can actually help to lower your heart rate. Read more about this here.
Smiling helps us connect to others. It makes us appear more approachable with more of an upbeat personality, whether we like it or not. For example, because women tend to smile more than men, when we don't, we can actually be perceived as 'cold' or 'depressed'. See article here. Not that other people's opinions should matter, but smiling at strangers acknowledges that they exist and that can make us (as a community), feel happier. Read about that here.
Smiling actually makes us feel good! By turning that frown upside-down, our bodies release happy hormones called endorphins, which is why it can help us manage stress (see above)! It also increases our pain tolerance, improves our focus, make us happier and more relaxed.
Smiling makes us appear more intelligent, LOLOL - according to this study. Note the keyword is that a smiling person is "perceived to be more intelligent", they might not actually be more intelligent, LOL.
Smiling can actually be heard! Here's a study to take a look at. It's a marvel that people can tell whether you're smiling or not - just by hearing how it changes your frequencies and what not. How is this useful? Not sure yet, but I suppose it helps us detect when someone might be lying to us or hiding something. Whelp, that took an interesting turn!