The third rewrite is the easiest. You're just going through, copy-editing, trying to make sure that you don't embarrass yourself when you let somebody read your work. Maybe you make a few last-minute changes, but really, this is where you have to decide on a final form and then let yourself walk away.
The second rewrite, though... That one's tough. That's where you have to think about every scene. What is this scene trying to accomplish? Is the structure working? Is this a Scene or a Sequel, and how does that affect the way the thing ought to be structured.
Still, I've gotta admit that it's necessary. Failing in the second rewrite will leave you with fiction that's barely readable, even if your story and plot structure are both working reasonably well.