Dean is at a low, low place. He doesn't have any reason to believe that Sammy will ever make it out of hell. He just has one reason to live -- to fulfill the promise Sammy wanted him to, to try for normal with Lisa. That's the one thing he's holding onto right now. D: Luckily, she's a strong, strong woman, and she knows Dean as more than just a pretty face. So I'm cautiously optimistic!