Open Question: Why do black women typically have nasty attitudes?

I work at a pancake house, and i see people of all races. I have had some bad experiences with black women, so i've learned to keep my distance so i dont get burned again. I've tried to overcome that by being nice to the black customers at my work. I treat them like anyone else--with a sincere smile and manners. But why is it that when i expect them to be generally nice, that 80% of the time, i get attitude back? I told a black group that i would have to prepare a table for them, and one said "yea you'd better!". I've also been told by a black women i need to check my attitude after i said have a nice day. I've also had people get pissed and demand refunds when their pie crust is broken (from different black people at different dates and times) I would appreciate some justification. also, i have had a lot more negative encounters with black women than with white. I have never had a white women attack me directly. one of my best friends is black. I love her to death, but i just dont understand why the majority seem so conceited when i didnt even do anything. I treat them like everyone else until they start fitting the stereotype. Then i know to avoid them, cuz i've been burned before. i work at a pancake house because i'm still in highschool, not becuase i'm "racist trash." I treat everyone the same, i just notice trends. That doesnt make me racist.