Because I like having control over who lives and who dies? Oh wait, everyone gave that answer, damn.

I do it because it's my favorite type of play in MMOs. Tanking has always made me a nervous wreck in this game, and similar MMOs. DPS on the other hand I either find boring in many games, or stressful in its own way because I know others around me are watching my rotation, numbers, etc... I don't play this game to feel stress. While healing may be stressful at times, overall I find it to be a much more calming and therefore fun experience.