This is absolutely just brainspew in word form. I dunno why I'm even posting this.
Sam Winchester is Lucifer's vessel. His only vessel. Sam, Dean, just about everyone knew this now. But only one person was absolutely certain that Sam would accept the position in the Apocalypse: Lucifer himself. And he knew it would happen in Detroit.
You see, Sam, he was bred for this purpose, it was his destiny... et cetera. But Lucifer had his other reasons for knowing this: Dean and John and Bobby and Castiel.
Sam was a heart-on-his sleeve kind of guy, usually. Lucifer liked that about him a bit. However, it made him fragile and malleable to his surroundings. Dean, Bobby, John, Cass... they were all the same. Hardheaded and unfeeling, so unlike Sam. And when they found out about the demon blood, and then the whole vessel mess... Well, they just didn't look at Sam the same anymore. And Sam noticed. Oh, did Sammy notice. And it ached, and it stung and it hurt Sam right down to the core.
And Sam didn't want anyone to notice, the sweet thing. So he, of course, took it on those huge shoulders of his and held his breath and bit his tongue.
Sam tried not to let on about it, but he was terrified. He saw what Lucifer was doing to his current vessel, an untrue vessel, now just a withering husk and a shell of a man. And he was running from that fear. Like a worm crawling from a bird. Much too slow and in the wrong direction.
Sammy was afraid of something else. He was afraid of the hunger and thirst clawing at him from inside his very own head. Sometimes, he saw things that weren't quite there, heard things in the whispers in the winds that he shouldn't. It was amusing to watch, honestly. The sparks of fear that pass in a second.
He kept it to himself, though, like a good boy. Just kept shouldering that burden, afraid to breathe, afraid to speak. Terrified by his own thought and feelings and cravings. He just kept crawling.
Occasionally Sam would fall off the wagon. If Dean wasn't looking he's lick and nip at demon blood on his hand or his knife. Never enough to do anything drastic, just enough to put a damper on the party inside his head. It never lasted long and before he knew it, he was partaking of the sweet again.
The scratching never really stopped, it merely lessened from time to time, when he was being a good little addict. Never telling anyone anything. Keeping it all to himself.
Eventually, he's going to break. He'll say yes because he can't trust anyone, because Lucifer won't cast him away.
But only Sam knew he would jump right into that cage and kill them both.