I've lost interest in SPN fanfic of late so I don't actually know what this is. I just got really annoyed after the episode and was like, "I'm gonna conclude this crap by myself" (re: Sam's whole wall breaking storyline). It's probably more Sam and Dean-ish that the episode was but with good reason.
It's unbeta'd so sorry for mistakes!
If anything, it's all over far too quickly. No more Lucifer, no fire behind his eyes; just stillness. The quietness throws him off, it feels lonely and isolated. But worst of all it leaves him with his own thoughts. They should be full of relief, full of gratitude. But there's that nagging tug inside his head that tells him that maybe he should have died in that hospital. Maybe all the constant dying and near-death situations mean something. Remaining alive is just going to expose him to more pain, more regrets and more failures. He's stuck in a world that he died in order to save but, it's almost as though it's self-destructive anyway. Sam wonders what the point is, wonders why he and Dean are trying to save it all over again.
But deep down inside he knows the exact reason. They do it because it means that they're saving each other. So while he'd been ready to give up, been ready to just take his penance, he knows that Dean wouldn't have given in so easily. He knows that Dean would find a way to save him from his predicament; because that's the Winchester way. Sam knows that he'd have done the exact thing, hell he had before. An innocent man had died back then and now Cas was stuck with Sam's hell rattling around in his head. As much as it pained him, he can't help feeling relieved. He has a brother that is willing do whatever it takes to save him, and he knows that he'll always do the same without hesitation.
Sure it's crazy and messed up but he gets the feeling that as long as fate allows it, they'll never change.