Why do I always make a mean face at people I don't know?

I've noticed before that when I'm out somewhere people make really mean faces at me! I know that probably sounds really stupid but it's true, lol. I mean, I don't think it's because I'm unattractive or anything like that, but I've been wondering why this is happening for a long time! Then yesterday I went to a concert and my boyfriend was talking to one of his friends. I didn't say anything to his friend because I don't know him very well. When I got home I posted a status on facebook about the show and my boyfriends friend commented asking me why I didn't say hi. I replied saying that he didn't say hi to me. Then he answered saying that he was going to say hi, but that I was making a face like "what the hell is he looking at." This got me thinking, I must make reaaaaally mean faces at people without noticing! I mean i'm really friendly once you get to know me, but I'm pretty shy and awkward when I first meet someone. I would love to have more friends and I think this could be possible if I weren't so awkward around people! Do you have any tips on how to be less shy and stop making that mean face! I want to be approachable but I feel that if I go around smiling at people I'll creep them out! Haha, so do you have any tips? I would appreciate them greatly! Oh, and I should probably mention that I'm just 13.