Before this gets ugly (ane I'm sure it will), here's my too simplistic answer to his question.
From wikipedia, entry "Denial of Death," by Becker:
The main theme of The Denial of Death is that most human activity ultimately concerns the denial of one's mortality. The full realization of one's own mortality is unbearable, absolutely terrifying and horrific. Since man has a dualistic nature consisting of a physical self and a spiritual self, man is able to transcend the problem of mortality through the concept of heroism, a concept involving his spiritual half. By being heroic, man feels he has meaning, a purpose, something that will never die, compared to his body that will die someday. One can be a hero to the eye of God, to the State, to the eyes of his peers, to his family, etc
Another aspect of this "denial" is the necessity of condemning what is foreign or unnatural, as seen in xenophobia or homophobia. It affirms our system of beliefs, and allows us to further the denial. So why do we hate? As usual, it's because we're afraid of what we don't understand.