Why are some people so prejudiced?

When I walk down the halls at school there are certain people that stand out to me as being different. Not necessarily in a bad way, but not really in a good way either. They're obviously nice enough to have their own friends, yet creepy enough that the "popular" kids stay away from them. Why is that? Why do people make such sudden judgements and stay away from other people that are "different." I think that the main reason is personality. I mean seriously who wants to spend the day with someone whom they have absolutely nothing in common with. You'd be surprised what you have in common with someone who appears to be way different than you. In junior high I had one of those built up prejudices against someone who turned out to be a pretty good friend of mine. She appeared to be the exact opposite of everything that I wanted to be in contact with, but one of my friends happened to be friends with her. So when all three of us had a class together and those two were communicating, I had either the option of being the class "loser," or actually trying to get to know her. I decided to start talking to her and found out that she had had a difficult life and was actually a really nice person. That just goes to show that if you misjudge someone, you could be missing out on a really great friendship.