What motivates people to cooperate

Rheingold mentions the cooperation of the masses for public goods. He refers to the development of Internet which is one of the successes of this cooperation. Today, utilizing this cooperation becomes trend. Open source, You Tube, Wikipedia, and Amazon's customer reviews are good examples. However, the success of these companies brought me a question. What motivates people to cooperate?

In fact, I can't find answer on my own. I can come up with several ideas, such as pursuit of self-benefit, intellectual satisfaction, good reputation and admiration, but none of them can convince me.
In many cases, the cooperation relates to their own benefit. For example, if someone develops a program based on open source, which means the program code opened to everyone for free, it can be for self-interest because that program can bring huge money to him or her.
However, self-interest alone can't explain the case of Wikipedia because posting something on Wiki can't bring any money. In Wiki's case, it may relate to intellectual satisfaction because if one's post is reflected on the pages without causing any argument, it means that many intelligent people accept his or her posting.
In Amazon's case, the desire for good reputation or admiration may be related. Amazon's customer review is also reviewed by users (You always see "X of X people found the following review helpful" in Amazon, right?). In addition, there is a ranking system called "Top Reviewers". These systems will bring reviewers a sense of admiration.

Yes, I could come up with these ideas, but I can't still be comfortable with the conclusion that these factors motivate them to cooperate. I can't understand why the quality of Wiki can be better than Encyclopedia Britannica (and also why many professors don't like student referring to Wiki!). So if you have any idea, please feel free to comment.