Why does this happen?

Lately I've been noticing a lot of people are very judgmental. I don't get why? If you don't know the person why should you judge them?
And its frustrating, because you have people calling you "stuck up" and what not.
And the sad part is they don't know one thing about you, for example me, I always get judged,
its either how I dress, or they way I look. Honestly hang out with me, and get to know me. I'm one of the nicest people to meet, and pretty funny. I don't like getting judge and I doubt you do like to, my moto is [get to know me, than say what you want to say] not just see me and start from there.
I will always remember when we were in school, I was at cheer practice and like I wasn't with any of my usual friends, and like I ended up talking to a girl there named Chelsea, at first she didn't like me (there we go again, by judging by the way I look) and we had started talking and than laughing,
by the end she had said "Your actually pretty cool" :)

-If you don't know a person, don't judge them because before you know it your gonna get judge and your not gonna like it.