What is the reason we hate some people?

Be prepared for many responses on this one.  I think that
the fundamental difference issue brought out by the last response is a strong one.  I
might also suggest that some experiences with others trigger our past experiences from
our own subjective notion that might be displaced on another.  For example, if person A
had a bad experience with a particular group of people, it's person B's misfortune to
happen to be of the same group as they interact with person A.  The hatred present has
nothing to do with person B, but rather the psychological background and baggage of
person B.  To borrow a materialist viewpoint, sometimes the aspect of economic wealth
and the lack of material prosperity might compel one person to dislike or hate another.
 Resentment over time about one's own personal economic situation and the perceived
threat that comes from another might be another reason one holds hatred towards another.
 In no way do these explanations justify the disdain present, but rather attempt to
explain it.