I like to solve puzzles; most of my contributions to OSS is in the form of bug fixing, which comes about because I've come across the bug myself, decided that it needs to be fixed so that gives me a chance to solve the puzzle. I see myself as a reasonable coder so I should be able to solve most problems, and the problems I can't solve at least my investigation can provide a top-notch bug report for other maintainers can skip some of the tedious investigation.

I don't find that this is just an OSS motive to me though as even in closed-source I'll often find myself in disassemblers (in .NET) or JS files, etc trying to work out how something works so I can provide the best feedback I can.

The other motive is the "I know what it's like" feeling being a coordinator on a few projects myself. I know what it's like to have people say "what doesn't it do X" or "why are you doing Y", it's generally because I don't have time, so if I'm on the other side I am more than happy to try and solve that problem myself. Again this is somewhat related to my first point but really it's a bit more than that. Just because I need a product to do XYZ doesn't mean that that is what everyone wants to do, or what the other contributors are in a position to make it do, so if I've got the skills why don't I tackle it myself?

Lastly there's the exposure aspect of it. As much as I'd love to think that everything I do for OSS is altruistic in nature it's not. I like that when I go for a job I can put on my CV that I've contributed to projects so a prospective employer can see my skills as well as just relying on my word. I like that github handle/ twitter handle/ bitbucket handle/ etc get recognised by members of the community and that people can see my as an authority on what I present about, blog about or yammer mindlessly on twitter about.