That's what a polite smile looks like. Polite smiles usually look unnatural because people don't use the same muscles to produce them, that's how we can tell the difference between people who smile politely and those who smile genuinely.
But it's irrelevant how natural people look anyway, we're not in kindergarten anymore, are we? Why are you still busy criticizing people for how they look like? I think you're more mature than that now, so I propose acting more mature, otherwise people will have reasons to treat you as such.
But of course that's your choice, no one can force you to act as a mature person.