for example one of my side projects is a webapp I have been building since september 2009, at which time I worked on it very intensely. Then I left it for a good few months, to only pick it up again when I was reminded that the problem it would solve was still a very real one - this spurred me on to work on it for another three months or so. I haven't picked it up again in the last two months.
I think I need real people around me to make the problem appear much more real, and keep me motivated. I think that is my rationale for this approach.