I read the question in two parts.
The caption asks what is your motivation for contributing?
The question in the post asks what is your motivation for being here?
I see those as different. People can be here, read only and not contribute. If your motivation for being here is to learn, you don't have to post and you don't have to express opinions or try to help.
So I see posting as starting with a different motivation that just wanting to learn (unless all you post are questions).
I read to learn, I post to try to give something back. It seems to me most others are similarly motivated.
I also feel I see a few posts which are made to let the poster feel good about himself; make him feel he is such an authority; such a "big man" on the forum; self aggrandizement. It is interesting to see that personality trial expressed at times. On some other forums, not this one, I see some people posting mean spirited comments about other people; let their "mean man" out. Anonymity can often encourage people to express their negative traits. I am surprised I have not seen more negative traits expressed here. To me that is important. I so dislike seeing people abused that I tend to stop visiting forums in which one or more members is often posting mean spirited comments about others and not being slapped down by moderators for it.