Theory #4 - Ego / Power Trip
Now this theory is meaty. I'm having a hard time explaining this though, because it's just slightly different than a few of the upcoming ones. This gist of the ego / power trip theory is that drivers in the left lane feel entitled to be there. For them, the world exists of only one person and the rest of the drivers are simply a swarm of annoyance.
You can recognize these drivers when you pull to the right to pass them. When you get about even with them they suddenly find the accelerator and speed up to prevent you from passing them. There could be a line of cars 5 miles long behind them, but as long as they are leading the pack their egos are happy.
I think the bottom line here is that the people who fall into this group care about only one thing, themselves. (I bet if you ask most people who are annoyed by left-lane drivers they will cite this as the reason for the behavior.)
Alas, people are complicated creatures. While this may explain some of the behavior there are always other reasons.
Next up, Theory #5 - Social Justice.