Since I can't speak for all of you, here's what I root for:
1) Basketball
2) The Celtics
3) Individual players
First and foremost, I love basketball. This game *has* me, and I will always defend my statement that basketball is the most fun sport to watch, follow and play.
Secondly there's the Celtics, but since that's the answer to your question, I'll talk about this last.
Thirdly, there's individual players that I root for. Dwyane Wade, Steve Nash, Joachim Noah, there's a whole bunch of players that I love to watch, as well as a lot that I hate to watch (Manu G, Paul G, etc) and I can definitely root for them at times.
So, about the Celtics... when I root for the Celtics, I root for a history of team-play, Ubuntu, for hard work, gritty defense and unselfishness. I root for Bird, Cousy, Russell, the spirits of those who used to be great. I root for 17 banners and counting, I root for James Posey, Powe, Cassell and all the other 'fringe' players that helped us get there. I root for an elite society, a dynasty, a green force.
So, yeah, I root for the Celtics.