I know why I play dps -- I was good at it. Most people here seem to think dpsing is an easy nobrain thing, and although I am new to this game In the other game I played a bad dps was just as bad as a bad healer or a bad tank, you were just lucky that the other DPS could pickup his slack.
However, I really liked being able to tank when needed. Although I geared and played a DPS I had side gear and a sidespec for tanking, and did that quite well. Usually I did it to get groups faster.
All that being said, being a DPS is more fun for me. Being a tank is me managing cooldowns and making sure the boss position is good etc. I did healing once and A. Sucked / B. Hated it. And as for DPS whenever I'm solo playing I like killing things as fast as possible! Also, there's kind of a competition when you dps. I suppose it's same for healers, but not really tank. As a dps you are paired with so many other DPS and whoever does the most damage gets the personal satisfaction of knowing they did the most damage, and being the big egotistical arrogant person I am I thoroughly enjoy, more than anything, being #1.