"I don't have a fucking choice!" Dean snapped. "I saw what happens when Lucifer wins. I saw the way things go down. Yeah, Hell sucks. I get that. I know from personal experience. But this isn't about Sam, okay? I have to let him go."

Dean would spend his whole life fighting for Sam if Sam hadn't taken that away. He knew the person he would become if he did. He needed to take the out while he had it, and if that included talking Faith out of it in the process, then so be it.