In the later stages, they all seem to lack fun for a time, but there has to be some compelling reason to keep going or it's never going to get done. Maybe obsession with the characters takes over after a certain point? There's a patch in every novel where not even that's enough and it takes sheer bloody-mindedness to finish the damned thing, but that kind of mindset can't be a long term thing. There have to be other reasons--fun, obsession, whatever--or it's just an exercise in masochism.