Why do people on the web act rude and rather mean to others who they don't even know?

I have often wondered why some people on the web feel it is necessary to be rude, insensitive towards others' feelings, and just down right mean to other people on the world wide web. If people talk to each other face to face in person they tend to show more respect or even don't say anything. Why can't people act this way even on the web? If anonymity is the cause behind this behavior then that would mean that those who act rude on the web are cowards in reality since they wouldn't act this way offline.