Isn’t it the best way to show your emotions? Smiles are universally recognised as an expression of happiness and joy and maybe there are neurobiological explanation to it, but they haven’t been found yet. In the meantime, just smile as much as possible because it’s so good to you! Did you know that even if you are in a bad mood and you force yourself to smile, you will feel better?!
Very interesting question; I just answered a similar one about laughing. As far as I know, there’s no evidence that any other animal smiles – some make funny faces that look like a smile, but as far as anyone knows, it’s not the same as a human smile. In fact some animals, like gorillas, make a face that looks a bit like a smile when they want to scare another gorilla away.
So why humans smile is really interesting, but I’m afraid I don’t have an answer for you.