Why do I hate to smile?

I'm 13 (in case it makes a difference) and I hate to smile. Don't take me wrong, I love being happy but I hate to smile. I don't know why. I'm just guessing it's because a lot of people comment on my one dimple and it's annoying but I don't know. I just hate to smile, for pictures, videos, even if someone tells me a joke, I cover my mouth so nobody will see m smiling. I have no clue why, I just hate it! Any specific reason for this you think?