Why do we do it? Because smiling tricks your brain into thinking you're happy, producing neurochemicals that actually make you happy.
That's not the only benefit. Have you heard the expression, "If you smile, the world smiles with you?" People respond more positively to you if you smile, even if they don't intend to. In The Happiness Advantage, Shawn Achor explains an experiment he has done hundreds of times in corporate settings across the world, in which he instructs half the people in the room "For the next seven seconds, no matter what your partner says or does, I want you to show absolutely no emotional reaction." Their partners then simply look those people in the eyes and smile and them genuinely. "Virtually no one can refrain from returning their partner's smile, and most break into laughter almost immediately."
And smiling makes you smarter. As Dr. Barbara Fredrickson reports in Positivity, your smile "quite literally opens you. In the moment you're smiling, you're more receptive, more able to see the big picture."
What else (that's legal and free) can make you feel better, make other people happier, and make you smarter?