i see where your coming from, but sometimes it is a confidence issue. I myself smile, as I now have a good set of teeth lol. Prior to that I wouldn't have really smiled as I had braces. Got them removed 5 years ago, ever since smiling away haha
but yea it's a confidence issue the majority of the time!
I think it's mostly to do with confidence issues and being shy. I don't have bad teeth or anything, but I don't smile much, because I feel akward smiling at strangers or people who I don't know very well. But people often mistake people who don't smile much as being stuck up. But on the whole, people who smile appear to be much friendlier and approachable than non-smiling people