Why doesn't he let go?

I've been going over this and I don't really understand why is he still holding onto the fish if he knows its bigger then the boat and he is getting weaker and he has all these cuts and hes bleeding he cant move and I just dont get why cant he just let go?? I mean I know its a good fish and he can get a lot of money from it and in other words is a "great idea" but if its such a great idea why cant he just grasp onto it? I was thinking maybe its not an idea, maybe its his life that hes trying to grasp onto since he was depressed hes trying to get a hold of his life and get it back to how it was and to start writing again?