Personally I think it's a whole lotta things, and not necessarily in this order:

1) ignorance, drivers just don't know that bikes are traffic. Like the driver of the huge SUV who pulls along side of me on an overpass (my only option to get from point A to B) and yells in my ear "you're not supposed to be on the road".

2) they have been ticked off by cyclists who do not ride safely and then... they see you. I say to friends who ask me for advice on cycling "please ride safely and legaly, the driver you tick off will not hit you, they'll hit ME because I'm out there more often".

3) Angry at life in general. Driver is angry, upset, they feel and maybe rightly so that their life is a wreck, they get inside tons of steel and see us. They are not thinking at that moment "hey, maybe I should get outside and enjoy G**'s green earth", they are not thinking at all.

We...are....handy...helpless and available. Please watch out for them.

4) Super commuters, folks who drive sometimes hours each day to and from work. All they want to do is get outta the car. Sometimes I see them glaring at me on my way to work. They are stuck in a car, on the way to a job they might not enjoy, and I look like I'm having fun (I am!).

I've found that I get treated better on a bike to work wearing plain jerseys, the fun ones like the Iguana jersey ("Iguana ride my bike") I get cut off etc more often.

Hey, have you thought we're not the problem, we may be your solution?

there's more but....I gotta get to work