Sam was drowning; both literally and figuratively.
Sam's a freak. He knows that and, after everything that happened with Ruby and the demon blood, Dean finally does too. Not that they actually talk about it or confront the issue. No, they wait around for Sam to be kidnapped and tortured before they do that.
Dean had figured he knew how his day was going to go. Do some research, hunt a little, maybe even go out for drinks afterward. He hadn't counted on taking part in someone else's quest for revenge, nor on Sam being held hostage to ensure that he would complete said revenge.
Sam is unexpectedly forced to take Dean's place on a hunt. Teen!Chesters