Dean's and Sam's world had always been each other, nigh to the exclusion of everyone else. Not even Bobby and Castiel came close to what they were willing to risk:
Hellfire and damnation and death time and time again.
Though each was equally as loath to bury the other, sacrifice was tantamount to I love you, and probably always would be. That was enough for them.
They cared and loved and sacrificed for others, but never to the death. Maybe their world was just too small. Maybe they'd just never had enough blood to make it bigger.
Bobby often wondered if the boys' world was capable of growing. Sometimes, he couldn't help but wonder if Adam had lived, would their hearts have been able to grow to encompass their little brother? Dean and Sam never spoke of the boy, and he questioned if it was because the grief was too raw, the failure and shame too sharp, but wasn't sure he wanted the answer.
They may have loved Adam, but not enough to die for him now.
-o-
Adam's world had been his mother.
But the secret Adam took with him to the Cage? She wasn't the reason he said yes.