(THIS IS JUST A THEORY)
Being hateful is built inherently into the game mechanically. The reason is as follows, it is a team game, where you do not have any control over the outcome of the game individually. If you are playing very well, then the rest of your team can very easily screw you over, so you naturally blame everything on the team. It's also a game where if one of your teammates suck, then they don't just hinder your team, they actively help the other team by giving them experience and gold. As a result you inherently become hostile and suspicious of everyone you ever play with, and that results in massive hate-fests.
- Quote from Totalbiscuit
I would tend to agree, but that's just me