Comment by kkoncevicius

I am writing to add two other possibilities.

Within myself I notice that the project becomes boring when there is nothing new left to be learned from it. Depending on the project this could happen at 50% completion or 90% completion. Take scientific research for example. For me there is a lot of motivation to figure things out, to fill the gaps, to make sure everything is solid. But then there is the mundane part of putting it into text and publishing. And my energy is not in there. I already know what will go into that paper, I know getting it out will count as "success" and I know it should be shared. But my libido is not in it.

Another thing - the end of a big project signifies a big change. If you worked on something for a long time, what will you do once it's finished? Norman Finkelstein in one of his interview put it like that (paraphrasing): "I think some people genuinely don't want to end the conflict [between Israel and Palestine] because they built their whole life around it. In the past it was a problem for me as well. I have spent my whole academic career writing about this conflict. I read enough books to fill this room. Literally. If the conflict ends tomorrow - what am I going to do with my life?".