The best reason to finish a project is that then all your other projects have a possibility of coming into existence. They are hiding beneath what you are working on right now, like those 9/10ths of the iceberg.
You might think you know what your next project will be, but it will change shape once you finish the one you are completing now.
I thought I would write a book about Gamoneda, but instead I wrote another one about Lorca. At first I thought this book on Lorca would be in Spanish; instead it is in English. At once point it was going to be two books, one about late modernism (including Gamoneda) and the other about Lorca's performative poetics, now it combines these two ideas. So that is why five years after Apocryphal Lorca I still don't have this book done.