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Sam says yes in Detroit, and in the space of a single syllable, there's nothing left in Heaven or on Earth for Dean to love. The grief tears him to shreds, threatens to unmake him, until Cas—fragile, human Cas, with a wrenching need to fill the void inside himself Heaven left behind—is there at Dean’s side, silent and still. He offers up nothing more than his body in return for a chance to pretend.
Dean fucks him the first time because anything at all is better than the ringing ache of his loss. Cas doesn't seem to care that it’s a detached, abortive thing, that he’s only a stand-in for a dead man. And Dean doesn't tell him, but Cas knows anyway: that of all the bodies in camp, his is the only one left that remembers the touch of Sam Winchester—a hug, a hand on the shoulder, the bloody press of a fist. That’s why Dean chooses him; that’s why Dean fucks him like he's looking for the last remnants of his brother's memory, as if he might excavate them with every greedy press of his mouth.
Reality settles between them. Cas knows he was never Dean's first choice, but he loves this broken, violent man so much that it doesn't matter, more the fool is he. The hurt is part of the package, and he likes the way its rough edges feel inside him when he chooses to stay even though he knows Dean wouldn't do the same for him. It gives him purpose; it makes him feel holy again.
By the time Zachariah’s Dean appears, they've existed at this conjunction of fantasy and necessity for years, become almost domestic—as domestic as anyone can be at The End of it All.
And when the Dean-out-of-time watches Castiel pull the mirror of his own body close and kiss him, almost chaste, on the corner of the mouth, mumbling words of halting reassurance, he's not really surprised. It’s hard to hide from the truth of yourself when it’s played out before you.
He can see it all unspool, every second of it, all the choices leading here. He feels the bottom drop out from his own belly when he thinks of his baby brother inhabited by a monster, locked for eternity in a cage of madness. He understands so intimately that he knows he would do the same, that he will do the same.
So when his own haunted eyes look back at him and there’s an emotion behind them that he cannot begin to comprehend, when his voice, hollow and torn, echoes back to him, saying, "You have to stop it. Don't let this happen again," he knows better than to argue.
And Cas does, too. He just looks down at a man who never really belonged to him, even here, and accepts.