It's always fun to start writing a chapter, especially if it is on a particularly interesting topic. Finishing the chapter can be a bit harder - sounds like a software project, right?
Finishing involves lots of details: rewording, double checking equations, actually doing TODOs (or sometimes just removing them), etc. After reading and writing so much about a topic, I want to move on to the next chapter - but I can't, not until I finish what I started.
A tip that helps me finish a chapter is I tell people when I'm going to finish it. In particular, I email potential reviewers asking them if they are interested in reviewing it, and telling them when I expect to be done. I have to finish it now; I can't ask these kind people to do me a favor, and not follow through.
This tip doesn't work magic, but it helps keep me on track. I can't expect to write 30 pages overnight just because I told someone I would. But if I'm 2/3 through a chapter, I can predict how long it will take to finish at a reasonable pace, and tell reviewers to help ensure I keep that pace.