Why do we respond to people in the way that we do?
Researchers in the field of dream symbolism theorize that the people in dreams do not represent the people themselves, but rather the part of the self associated with those people. For example, if one dreams of their father, the parts of the self accentuated are quite possibly the paternal, protective, providential aspects.
If you claim to hate someone, you really only hate yourself.
This is quite a powerful theory. It means that the subconscious recognizes others as extensions of the self, rather than as isolated entities. In other words, the people you know aid in resurfacing or accentuating certain characteristics that you yourself hold.
Thus, if someone reminds you of a characteristic that you do not like of yourself, what is to stop you from disliking that person? If you claim to hate someone, you really only hate yourself.