And Dean had a bad habit of taking blame and guilt on himself too.

"Cas was saying it was our last and only chance. I... I'd sooner say yes myself than let that happen to my brother. And Cas... he's human now. No more mojo, and he thought he was dead at first." Fear gripped as he realized what that would mean. Sam gone, Cas gone. If Bobby was gone too, he'd be all alone. Everybody would have left him, just as he always feared the most.