When you start your novel, there is the possibility that: the end result will be nothing like what you intended to write.
That the copy-edit you thought was going to be the final draft will turn into one more rewrite.
That the beginning and middle give you no trouble at all, but the ending succumbs to the same uncertainties that plague your characters, and can’t decide which road to take.
That long before you are ready to say ‘Enough! It’s done,’ you will be thoroughly sick of it and desperately anxious to start something new.