Why are some people so ignorant and close minded when it comes to race?

I'm a black woman and I'm always stereotyped as loud and angry before people even talk to me...but I am anything but that. I'm quiet, I speak well and I get straight A's in college. But when people get to know me, they say I'm trying to be white. It's like no matter what I say or do, they have to put something negative on it. Why? It happens mostly in my romantic life as well because the guys see me and they write me off as the "stereotypical" black woman but the ones who do get to know me reject me because "I'm too white" Why are people like this?