As my professional life gets busier and busier, my reason for coming change, however on average they are:
Helping others, and doing something back to the community that helped me (especially so with the R community).
Moderation. I do my part to keep the forum clean.
The best way to learn is to help others, I often find that explaining concepts to others is the best way to grasp a concept yourself. You might think you know all about something until you try teaching it to someone else. Dason is right: "the more you learn the more you realize that you know nothing" - isnt that great? I love learning new things. I love to better myself - this is something that motivates the best.
The open-source community, the free knowledge sharing, people helping with zero commercial interest really shows me the way forward for us as humans. I hope others will start realizing this. Here is a great example of what we are learning from such phenomenon as the above (the internet style) - as we start moving from the carrot and stick model:
(this will give the regulars insight on why they come here)
http://www.youtube.com/watch?v=u6XAPnuFjJc&feature=player_embedded
Why the open-source community works:
http://www.youtube.com/watch?v=Xe1TZaElTAs&feature=player_embedded
These videos also illustrate why the internet is so uniquely potent in spreading knowledge, and why so many people develop software for free and help for free. It also show why net-neutrality is key.
Now, as I dont have the luxury of procrastinating that much anymore, I usually just moderate and tackle questions that for some reason tickle my fancy (R & Ecology questions will do that often ;-) ).