Pleased to see monks respond with items not on the list. Keep them coming!

This is because I have no training in psychology and just kinda blindly lifted the original list from Motivation for online participation (wikipedia). Some more from Understanding the Motivation of Contribution in Online Communities (wayback machine pdf):

• Satisfying other members needs
• Enforcing service excellence (reminds me of repeated scolding and downvoting and reaping of certain infamous monks ;)
• Gaining a sense of helpfulness to others

The last one resonates with me and I remember Larry singling it out with this famous quote from Programming Perl:

the only personality trait Perl programmers have in common is that they're all pathologically helpful

Update: 1nickt was the first to suggest this with: I have a "compulsion to be helpful" (singling out davido)
... oh, and I see 1nickt's home page contains yet another possible motivation: Learning something at the Monastery every day