"I'd leave Dean there if I thought for a moment that Zachariah wouldn't make things even harder for Castiel if I did." Sam knows he can't stay there because his strength of will would break eventually and he'd knot Castiel. Maybe even give him the bonding bite, which would be difficult for them both.

He sighs, rubbing his face. "It's not that I don't want to. Every instinct I have is telling me to get in there and deal with his pain, make him mine. And...I don't know. Maybe I would be his choice, but until he can actually say it, I can't take that chance."