Michael Corleone: I feel I'm getting wiser now.
Kay Corleone: The sicker you get, the wiser you get, huh?
Michael Corleone: When I'm dead, I'm gonna be really smart.
Michael Corleone: Your enemies always get strong on what you leave behind.
Michael Corleone: Friends and money - oil and water.
Michael Corleone: Never let anyone know what you are thinking.
Michael Corleone: Never hate your enemies. It affects your judgment.
Michael Corleone: Just when I thought I was out, they pull me back in.
Answer: It shows you how greedy they are even as they've made it to the zenith of their criminal careers. That's why you see Michael pass it on disgustedly and stare blankly into the ether. Michael's goal was never to be the boss or capo di tutti capi, he only did it to protect his father and then his family, his main goal. And at that moment he realises he's never getting out. He's a business man in a world of criminals (which he is himself).