Disclaimer: Blah blah blah, generic disclaimer here.
Tag to 5x18 Point of No Return. Spoilers.
Only
"So," Sam started.
"So what?" Dean asked when he didn't finish the thought.
"I saw your eyes," Sam said after a moment. "You were totally rocking the yes back there. So, what changed your mind?"
Dean was quiet a moment. "Honestly? The damnedest thing. The world's ending, the walls are coming down around us. And I look over at you and all I can think is 'this stupid son of a bitch brought me here.' Just… didn't wanna let you down."
He doesn't mention that it had a lot to do with Cas, as well. A lot to do with the severe pulverizing he'd received from the Angel
I rebelled for this? So that you could surrender to them? I gave everything for you! And this is what you give me?
He had seen the hurt in Castiel's eyes, the pain of putting faith in someone who just gave up. Dean knew that feeling, and he knew Cas had suffered that same letdown twice, first from God, then from Dean so soon after. It was killing him. Dean couldn't let himself cause that pain anymore.
"Will I see you after this?" Dean asked as he carved the sigil into Cas' human skin.
Cas doesn't answer out loud, but dean could swear he heard the Angel's voice whisper in him mind.
Only if you don't say yes.