The laws regarding the usage of lanes seems to vary by state, which can lead to differing philosophies about freeway driving. 'Round these parts the left lane is not a designated "passing lane" and there is no law preventing passing on the right.
Pretty interesting table of "keep-right" laws by state here:
http://www.mit.edu/~jfc/right.html
Interesting driving habits I've noticed:
In the middle of nowhere, I set the cruise control and get going down a lane. I come upon a slightly slower car in the other lane and creep up as our distance closes. The other driver subconsciously matches my speed and either flanks me for miles or periodically zooms ahead and slows down until I creep up again. Again I'm using the cruise
I set the cruise control about 10MPH over the limit and camp in the left lane, passing car after car after car. Eventually someone comes flying up behind me and starts a mating ritual with my bumper. Once there's an opening on the right, I move over to let the hornball pass, but he just pulls up about 6' in front of me and decides that's where he wants to be. Now there's a slow moving semi in front of me and I want to get back to the left lane, but Speed Racer forces me to disconnect the cruise and slow down to get behind him. WTF?
Driving along at the speed limit in the left lane and periodically passing cars going 10 under on the right. There's sparse traffic. Someone in a hurry comes up behind me and there's plenty of space to pass on the right, but he'd rather ride my bumper instead. Why? Just pass on the right and we can both be happy
Some people drive like helicopter pilots except instead of flying from one forced landing site to the next they race from one bumper to the next. They don't seem to care about their speed as much as they enjoy tailgating someone in front of them.
Other people will invariably overtake a car to their right. They could be 10 car lengths back and I'm on cruise and change lanes. They will speed up and overtake me and then slow down and occupy that new spot 10 car lengths in the front.
A lot of people drive not by absolute speed but by relative position. If I subtly slow down or speed up 10mph, so will they.