Why do we "Hate" Those With Whom We Disagree?

In October I lost a friend of nearly eight years. He didn't die, but we had a major political disagreement.
He is very left wing, and I have a more right wing outlook on life for the most part. (Some have said I'm too right for the left, and too left for the right). My other friends accept this as we are all different people, but he flew off the handle (politics isn't something we spoke about until that time), threw some insults around and generally acted like a spoilt brat about everything.
I thought (naively, I might add) that since we were friends and he knew me, then he would be a man about it and we could move on. This wasn't to be. He completely cut me out of his life, so even if I wanted to smooth over the cracks I wouldn't be able to. It is his priority to have people in his life who he likes, but his hatred towards me over a disagreement was so over the top. Then again, this isn't the kind of thing a real friend would do.
After that incident I began examining my personal relationships and friendships a little bit closer and noticed how much I compromised my own beliefs to suit the mould of a few people who could "shout" a little louder than me. I have changed since then, for the better I think, so his hatred was actually a pretty good catalyst in a way, although horrible at the time.

Apart from that, I'm used to the old forum debates dressed up as arguments with the vilification and the bile spewing. It's easy to hate someone you don't know, which is why verbal cruelty abounds in this medium.