Maybe I can give some hints, based on my motives
I used to contribute to another well know site (experts?) a couple of years ago. But was disappointed with the general quality of answers. The reputation system did not match my expectations (no way to vote down, questions closed, for instance).
Doing some professional research, I first found SO (later SF) and I was impressed with the quality of answers and - more importantly - the match between that quality and the rating of a question / answer.
I found at the time a very useful answer and wanted to reward the person. So I created an account but discovered that a minimum of 15 rep was necessary to vote up. Very busy, I decided to give up at the time.
A few weeks later, I tried to give the answer I would have liked to get and the person was happy with that. So I got the 15 rep pretty quickly (but I forgot which user I wanted to reward initially!).
Then I wanted to be able to add comments to some questions / answers ; thus I needed 50 points...
Now, when I have some free time, I try to answer seriously a few questions, just for the fun of it, since I may need serious answers myself later...
I have to say, also, that the way the site is built, and run (free, opened moderation), how easy it is to write a q/a while viewing in real time the text, the convenience of the
* _ ** >and ``, the constant changes, contributes to the contribution-likeness. I like also the way one can view the profile, retrieve one's own data, by time period. And I like the tag system, highlighting the subjects of interest.
It looks like an hagiography! But lastly, I spent quite some time on SO/SF, so I thought there must be a reason!
What about yourself?