This is a repost of deborahSusan Clarkson's mail to the Summer of Code mentors list (reposted with permission, of course):
Having worked as a volunteer and with volunteers for the past 30 years, there is a myriad of reasons that motivate people to get involved. Most are motivated by a combination of factors.
Those motivated by passion, as Karoly mentioned, are willing to go above and beyond. Passion drives them to overcome their shortcomings, and they accomplish more than one might expect.