He's tired, so damned tired all the time.
It was so easy in the beginning, so simple to pick up the threads of the life he thought he'd left behind, to take to the roads and let the hunt guide his footsteps across the states.
But. But then came the visions. The deaths. The demon, the special kids, his death, Dean's deal. Hell. [ruby and blood and despair and sorrow and loss] and he's just so damned tired. Lucifer's face is painted on the insides of his eyelids, burned into his retinas and seared into every cell of his brain. Dean is becoming... other. He glows, ethereal but hardly delicate, the perfect combination of warrior and human, and he can see, can almost feel Michael's grace filling the hollow spots in Dean's soul.
He's so tired, so damned tired of struggling, of hurting, of being rejected and loathed and despised and all he wants, all he really wants, is to throw himself at the feet of the angels, at Dean's feet, to prostrate himself in the dirt and clutch at Dean's ankles and plead for Dean's grace to burn his soul clean.
Despair is a silver knife, is the blood-stained carton he pretends doesn't exist, is the expression on Castiel's face whenever his name is mentioned.
He's so tired, so damned tired, and when he says "yes", when he feels Lucifer take control of his stained, worthless soul, it's a relief to finally, finally, rest.
(and if, in the back of his mind, he can see his own future stretched out before him, see himself bled out and tormented for an eternity, well, Sam has long since gotten used to the sound of his own screams echoing in his mind.)