It was undeniable. The whole idea of it just popped into his head so naturally it scared him. And he does not get scared often.
Castiel blinked, although it was merely a reflex ('You'll find yourself imitating those Winchesters, baby bro'), and stared astonished on his charge.
It wasn't the sacrifices made, but the intent and honest vacancy of needing to be 'paid back'.
Castiel concluded that is why the thought is so easily available in his mind, why he would do one of the most difficult things he could.
He would rebel for this righteous man.
It was undeniable.