I actually find myself much more capable of finishing large, multi-step projects when they've got social implications riding on them. I enjoy my private projects more when I'm doing them, but have trouble finishing them if they take more than a few hours of serious work.

The first category does include things that others didn't directly tell me to do, though: there's also things I'm doing as favors to others, public-facing projects I came up with independently, and so on.