Theory #10, Keeping up with the Jones'
Well, so far all of the theories have had practical explanations. Quite simply, this one doesn't. The theory is that people just want to be cool, like everyone else. Like the Ego theory, somehow driving in the left lane conveys a level of status that people want to have. Driving a BMW, Hummer, or Mercedes is not enough if you aren't driving in the socially hip lane.
Maybe they just want to be part of the herd. Here's an article that could apply to this theory: Understanding the Herd Mentality
I really don't have much to comment on this theory, because if this is why people are driving in the left lane they are just stupid. There's a whole road to drive on to look cool. In fact, I've heard that the really cool people actually drive on the right side. Yeah, seriously. Tell your friends.