I was a bike messenger for a long time, bikes have a hard time sharing the road with vehicles with engines.
People treat you like dirt even when your trying your best to be conscientious.
For instance, most folks want bikes to stop at a red light, but imagine you're driving up hill and are behind a cyclist, if he stops for that red he would be going a whole lot slower.
I've had people yell at me for being on the road, even though I was at work! Its not like I was cruising for pleasure in downtown SF & NYC! I never go into peoples office and yell at them.