|the reward pathways in the brain|
The answer is always different but there are some common threads. When people ask for personal benefits, they want something tangible.
The power of publicly saying thank you is not to be underestimated. I thanked Besjana Osmenaj for the good work that she did and to my amazement I find it on Google as the sixth thing to her name.
translation memory based system. They will have to translate literally and localise it for an audience, they will need to learn the terminology for a particular subject matter. All this training can be done on Wikipedia articles. The reward is that for many languages it can result in their work being used and read.
A professional computer person may want to add MediaWiki as a platform that he supports. The LocalisationUpdate extension will help him improve the wikis in his language whenever people contribute at translatewiki.net. It is even an option to learn how to program MediaWiki or to learn internationalisation skills so that people seek out the professional for supporting localised software.
When we have a contest, we can provide the contestants with a certificate that explains the effort they put in and our gratitude for their contributions.
My point is that just saying "it" is fun does not work for everyone. Identifying skills that can be learned, skills that are valuable in real life may convince a person that there is more to it then a hobby for idle rich people.
PS there must be many more rewards, what would makes or made a difference for you ??