Just saw this thread whilst waiting to see if all my preferences will update.
Well... I've joined a morass of projects, because:
1) Any project, including the alphas and betas, can be useful... Even if just to say that the project as built won't work for what you want.
2) Any science is good science. I firmly believe in the advancement of all fields of study... Although my 'addiction' all started with the hopes of finding little green men in the stars.
3) Like Pletsch down there, I'm addicted to colors. Different reasons, though.

And like Chertsey, I love numbers and math. So I guess I'm a flock animal? Heh heh.
4) It's nice to be able to have my computer do something with its idle time while I'm waiting to do a show, or waiting for the game servers to come back up, or surfing my e-mail or for po... Oh... Kiddie-friendly, right.
5) And I think this is the overall 'big one': I like to feel like I'm part of the 'worldwide solution', rather than part of the problem. Sure, I might not see rapid results in reality... Heck, might not see them in my lifetime... But if I can help and be one of the millions working and pushing for that cure, or for that key to unlock the secrets of energy, or the stars, or whatever it is... For the benefit of humankind... I feel like I'll have left a permanent and important mark on the world. It might sound cliche, but it's the truth of the matter.
6) And running mildly counter to that last point... A little friendly credit or recognition competition (are you listening, Chertsey? You UOTD hog!

) never hurt anyone... It's all in good fun!
So... Yeah... That'sy story, and I'm sticking to it!