Everything that happened in this episode, everything Sam's said to him over the last few weeks, everything Sam's done for him since he went darkside, Dean finally can't help but understand just how much his brother loves him. The problem is, he can't handle it. He can't sleep, can't concentrate, can't come to terms with it, can't stop thinking about it. Sam thinks the Mark is getting worse, but that's not it at all. When Sam confronts Dean, expecting to hear that the Mark's effects are getting worse, Dean remembers what Charlie said and tells him the truth, for once.