Why do some people get road rage so easily?

A couple of weeks ago when I was on my way to work somebody stopped in the middle of the intersection to yell at another driver. When this happened I started wondering what in the world the poor person could have done to get yelled at. I mean seriously, when your driving down the road how many people do you see doing everything perfect. There's always somebody on the road doing something wrong. I personally think that the main reason people are so quick to anger out on the road is because they don't realize that they've probably done the exact same thing before. Granted, when somebody cuts you off you aren't going to automatically think of all the times that you cut someone else off, but you can control your anger. Obviously this is easier said than done. If people can control their anger on the road, then it can be a lot safer out there.