Grinning like a fool....

Along with all of the usual comments I get regarding the retro/vintage look I sport for the most part, I get frequent comments regarding my smile or lack there of. People always ask me what's wrong? Are you mad at me? Why aren't you smiling? Are you in a bad mood? Why are you so stuck up? The answers on most occasions are Nothing, no, I don't know, no and I'm not! Do I have to go around smiling like a mental patient in order to stop people from thinking I am a snob (which I hear all too often)? This has kept people from approaching me many times which in turn has hindered my making friends. Most people I do become friends with end up telling me that on their first impression of me they were either intimidated, thought i was a snob or *gasp* thought I was a bitch, but upon getting to know me have found the opposite to be true (for the most part ;). So how do I fix this without looking certifiable?