Rude people in cars

Why do they do it? Why is it some motorists and their passengers think it's funny or justified to verbally or physically assault bicyclists?

Tonight on my way from work to a yoga class, riding through my residential neighborhood, a passenger in a SUV felt the need to shout an obscenity out the window at me. Why? I have no idea. It was about 30 degrees out, so what drove this guy to roll down the window in a moving car just to be obnoxious to a cyclist?

And it's not the only time something like this has happened to me. I don't really keep track, but I'd guess that a couple times a month someone takes issue with me being on the road, or whatever it is that drives them to shout things. Very often it's something like "get on the sidewalk" or "get out of the road"... which, while annoying, I can chalk up to ignorance and frayed nerves from the traffic around here, along with the perception that I'm in the way. Tonight it was a simple shouted obscenity... you know the one often followed by "...you". And it wasn't the driver... it was the passenger? What's up with that?

Well, maybe it's just that weird mentality that views harassing others as fun. In the 30+ years I've been riding more or less seriously (well, seriously isn't the right word... I smile a lot), I've had a number of folk shout at me, a few throw trash out of their cars at me (including at least one beer bottle), and once even had a passenger lean way out to slap me with both hands on my back! And I just don't understand it. Anyone out there have an explanation?