A lot of hate is based on fear. A lot of fear is of the unknown, the different. We think - the people of that other tribe could be a threat to us, so lets get them before they get us. If we meet them and find that although they may have different ways of doing things, at heart they are just like us, they stop being a threat so we don't fear them, so we don't hate them.
Societies which try to keep their members away from other "contaminating" influences (other religions, no religion, democracy, communism, banned foods, skimpy clothes, equality of women, pop music) keep those other influences as threats in the eyes of their people, so they fear them, hate them, want to destroy them.