Preface: My primary method of transport is by bike if I can help it. I hate cars and everything they represent.
When I get behind the wheel of a car, it's like something in my brain flips and I notice my behaviour changes. I become more annoyed, aggravated, and impatient. I don't know what it is, but something about being surrounded by a steel cage that can explode up to 100km/h at a moment's notice makes you feel invincible.
A large part of the problem comes from the design of the road. To accommodate as many cars as possible the road is built to be gigantic, and when you're driving a car this makes you feel like you're safe to maneuver even at the expense of everybody else.
I think if we started designing cities properly, focusing first on pedestrians then bikes then buses etc, we'd end up with an urban environment where motorists do not feel as comfortable throwing their ego around and so will quiet down and behave.