Variable Speeds (on the freeway)

I like my cruise control. Whenever possible, I set the cruise, and I don't have to worry about my speed. That said, other drivers on the road seem to fluctuate their speed continuously! Some drive incredibly slow, until they hangup the cell phone and then they re-find the center of their lane and punch the accelerator.

My "favorite" is when I come up behind someone driving slower than I am and I move over a lane to pass. Nine out of ten times that person will speed up to try to keep me from passing. If I decide to go back behind them, Nine out of ten times they will slow down again. With some people, you have to significantly speed up to pass them! Is this similar to that driver Psyche that won't allow another car to merge in front of them (only behind)? Why are people so competitive about driving?