I have my reasons, Seoman. You see, in my experience, people typically choose a faction for one of three reasons.
1) One faction is perceived as being more powerful than the others. Power-gamers and the immature flock to this one.
2) One faction has more people than the other; more people join so that they'll be able to more easily find help. This is especially important (and prevalent) in PvP, where numbers really matter. This reason is the guiding light of sheep, twinks, and gankers, and was obvious during the huge surge of Alliance players during the notorious Hillsbrad gank-fest.
3) People prefer a given class or race and will play it regardless of other factors because they enjoy being what they are. THESE are the people I want to group with-- they are typically more polite, more fun, and less stressful to deal with. They are also in the minority. (Ironically, they also include some of the most skilled players in the game.)
Power-gamers, the immature, sheep, twinks, and gankers annoy me. Most of them will also choose their faction based on reasons 1 or 2. Thus, most of them will end up together in one faction or the other. By choosing the minority faction, I'll end up playing with the fun people more often.