1) You're a case of arrested development. You've spent your childhood glued to your video games. You think those other machines on the highway are occupied by cyborgs, instead of humans. You think those exploding cars will reconstruct in the next game and everyone will survive to play again. Since you can't make a human connection, you have no problem treating them like inanimate objects that you must pass or eliminate in order to "win."
2) You suffer from distance distortion. You can't judge your own position on the road in terms of centering in your lane or holding your space on the road. You require guidance from other drivers in order to get where you're going without driving off the road. The closer you get, the more secure you feel. You rely on them to keep you driving straight ahead and not careening off the side of the road.
3) You really are just an arrogant jerk. You think you own the road and everyone should get out of your way. The sea should part to make way for your grand, entitled passage.
So, if you are a tail-gater, which definition would you be proud to claim?