gustavolacerda points to an interesting discussion over here about what makes work "work" and play "play". For me there are two major things going on, I think. One is a very irrational but real feeling about projects that are "mine" being important, and projects that "belong" in some sense to other people representing drudgery. I've had experiences in both directions where something made me feel differently about my involvement in something — reclassifying it in my head as now "mine" or now "not mine" — and there followed a major difference in how good I felt about working on it. The other is that the external pressure to get something finished often lags behind my attention span having switched to deciding that another thing that I want to think about. In this case, if it were up to me, I would just ignore the old thing and play with/work on something new, but there are good (but external) reasons to finish research that I've started.