Why do people smile?

Why we smile? The easiest answer i can get is because its fun. Imagine if there's no smile in this world.

I have friend who always smile even when he's angry. He doesn't smile only when he got headace. I ask him about it and this is the answer, "I try to smile everytime as i breathing even when im sad. Smile is good for your health"

I'm not a medical student, so i don't know what is medical explanation for why smile is health. But my friend, he looks younger than his age, maybe because of his smile habit. He become funny person, nice, and happy person. He handle all the problem came to him with his patient smile. He's not a hurry person, he's life seem so relax even i know for sure, he's life is full of problems and his job is full of deadline, living in big city with expensive life and his middle salary....i think hurry and stress is normal, but not for him. He keeps smiling.

I try to do it, learn to smile as i breath and its fun. Everytime problems come to me, yes im stressed, yes im upset, but when i smile on it...it become easier. The secret is just, smile from heart, its not only lips that smiling, but come from the heart.