Are you asking about behavior in the Fluther context or generally? If on Fluther, I don’t know that compel is the right word at all (or if it is, then you’re probably talking about a very small number of people, special cases). I’m not even sure incentives is right, much less “powerful” ones. I’d choose moves or motivates and say “strongest motivations.” When you’re talking about “incentives,” you are talking about rewards and hence something pretty much outside what you’ll find here. There are no rewards here other than entertainment and a certain emotional response to the give-and-take.
Speaking for myself both on Fluther and off, I’d say it feels good to share what you know. It is nice to know that something you have (including knowledge and experience) is valued by others. I believe that many people teach, write books, lead and join self-help and support groups, counsel, and attempt to talk to their children out of similar motivations.
There is also a desire to fill a need, and when a need for help matches help I have to offer, there is no reason to withhold it.
And of course there is the fact that the transaction of giving and receiving help in the form of answers and comments is simply an occasion, an opportunity, for some human interaction, and I think that’s all some of us are really looking for here. That’s why the social aspect to so many questions is so strong.
For some I think there is also validation in persuading others to their point of view and reassurance in testing their experience and conclusions against those of others. Sometimes we find ourselves surprised and corrected instead. Then we are learning, I hope, and that is a good thing too.