His brother has made his choice, and now Sam is going to make his.
Because he is Sam. No matter what anyone else says, he is here, he thinks and he acts and he remembers everything that has been done to him.
He remembers a locked room. Restraints. Being sure he would die, alone. Screaming for Dean to help. Screaming. Screaming.
Dean has made it clear, he'll take a dead brother, or even a broken one, a soul torn to catatonic pieces, over a living one who can't be what he wants. Sam tried. He'd have taken his soul back if he knew there would still be a him afterwards.
Hell, part of him wants his soul back. He's a better hunter now. He's not sure he's a better Sam.
But it will destroy him. He is saying no. He is screaming no from the top of his lungs. He'll carve it into his own skin if he has to.
Dean is going to force his soul back inside, however Sam tries to fight him.
Because as he is, Sam is not enough for him.
He only has one option left. For his own sake, Sam will make himself uninhabitable.