hate is manifested when one person refuses to take responsibility for their own emotions and projects their feelings onto 'the hated one'. it is easier to look outside themselves as the world is a stage (and quite entertaining at times), than where the hate originated from, which may be an injustice the hater has experienced in the past, abuse, or just plain ignorance - maybe the 'hater' was actually ignored themselves at certain key points in their maturation. In short, ignorance. Self education is the way to go, educate yourself about your Self.
Hate can be caused by so many things like jealousy, wealth, power and religious or cultural differences. If people dispute against something, they are likely to develop differences that result to hatred.