Dean's not stupid.
He's been told, occasionally, that he and his brother have an extremely unhealthy relationship. He never gave it much thought. He always just thought, well, that's how we are, there's not much healthy about my life anyway. There were generally more urgent things to worry about.
There's a part of his brain which understands that normal people, when they lose family members, do not immediately try and doom themselves to an eternity of Hell to recover that family member; it is not, however, the part which is currently running the show. Dean can't conceive of a life without his brother. He stared at Sam's body on that table and he couldn't quite convince himself that it was real. He talked to Sam, hoping against hope that he'd hear an answer. He thought that at any second Sam would get up and just have been unconscious, or Sam would walk in laughing and the guy lying there would be someone else, some random tall man with girly hair, or…or something. There had to be something.
It had taken a while before some rational thought had battered its way through to Dean's mind, and he'd understood that if Sam was going to get off that table, Dean couldn't just sit around and wait for it. He had to make his own arrangements.
Now he's thinking, distantly, that he'd rather be dangerously dependent on his brother than have the dubious satisfaction of having had a really awesome relationship with someone he's never going to see again. He understands that this is stupid; he wouldn't argue with that. Sometimes you just have to do something. Sometimes you just have to do stupid things, because there's no choice, just because.
There's crossroads dirt gritted around and under his fingernails. It's dry, and scratchy, and makes him want to claw the skin off his fingers.
The demon smiles with white teeth, tilts her head and quirks her eyebrows in amusement, and Dean bargains away his life and his afterlife, because he has no other options. He can hear his own heartbeat, running too fast, as if it's got to fit the same number of beats into whatever time he has left, whatever he can beg from the demon. Ten years. Nine. Eight. His pulse is speeding up. Five.
One year, and one year only, the demon purrs, still smiling; she knows that Dean will pay any price she cares to name. What do you say?
She's standing too close to him. Making it easy.
He doesn't want to do this.
Sam. He thinks of Sam.
Dean abandons hope; kisses her, and seals the deal.