smiling is natural when you're happy or amused
fake smiling isn't natural, but it's not without purpose. smiling even when you're not happy can be a good thing
the act of smiling actually can make you feel happier. your brain associates a smile with happiness, and smiling can sort of "trick" your brain into thinking you're in a better mood, altering the chemicals in our brain a bit. and likewise, frowning, scowling, etc, can negatively affect your mood.
smiling also signals things to other people. if you're going around with a neutral expression or a mean looking resting face (like I have), it can send bad vibes. People may think you're upset, angry, unapproachable. if you're smiling, you're more friendly looking and inviting. would you rather go to the smiling cashier or the one who looks like she'd rather burn the place down than stand there five more minutes? most people would go for the smiling person. whether these faces reflect the inner emotions is irrelevant, because we can't really know what they're truly feeling. so similing is important.
people also tend to mirror each other. if you're around smiling people, you're more likely to feel happier or smile yourself. if you're in a room of scowling faces, regardless of if they're unhappy or not, you're probably not going to feel as happy or smile yourself.
i try to smile in public and in pictures because i do have a mean looking resting face. i can be perfectly happy but look like i want to die. i find i get a lot more positive reactions from others if i look happy than if i show no emotion.
fake smiling takes practice. it's not just about turning the corners of your mouth up. gotta work those muscles around the eyes to. be all like Tyra Banks and "Smile with your eyes", otherwise something will seem off, and it will obviously look fake