Oh God so I was re-watching Swan Song and I just realized something:
That look on Sam’s face there? That’s the moment when he’s finally realizing is that he’s been fighting with the wrong weapon. Sam thought he could overpower Lucifer with his demon-blood powers, but those are based on hate, on treachery, on evil, and you can’t defeat the devil with his own lifeblood. No; what ultimately overpowered Satan was the one thing he could never really understand: love. Not the selfish, jealous, possessive love that Lucifer knew, but a deep, utter, soul-abiding love. Love that was selfless, that knew no boundaries or limitations; love that said, ‘I need nothing more than for your soul to exist in this world and be happy.’ Love that would happily sacrifice itself, that would look up from the tormented pit of hell and smile because that sacrifice meant another had lived.
Sam Winchester is realizing here that the demon blood was a firecracker next to the nuclear weaponry he was really packing. And after he realized that, his grip on Lucifer never once faltered, because when it comes to love for his brother, Sam Winchester has that in fucking spades.