I haven't been able to distill it to first principles yet, but I do have a practice of writing code in such a way that it invites the next step.
I suspect that at first I did this in an attempt to hack my own sense of motivation, like putting the books you need to return next to the front door. But it turned out to be quite handy for seducing junior developers (and sometimes senior developers) into finishing an idea that you started.
They are so proud that they've thought of something you didn't think of, rather than something you were looking for a maintainer/free cycles for.