He’s in Hell, holding an instrument of control and torture in his hand because a demon told him to. The implications of this are not lost on Dean, though he chooses not to dwell on them for long. More important things at stake here, and all that.
Of course Crowley, the perceptive son of a bitch, won't let it go unnoticed. “It’s been a while since you’ve been here, hasn’t it?” He remarks in a polite, conversational manner that is contradicted by the gleeful smirk on his face. “So between us girls, Dean, does it bring back any memories?”
“It doesn’t bring back a damn thing, so shut up,” Dean snaps, ignoring Crowley’s skeptical look. It’s none of Crowley’s business. Besides, he’s telling the truth.
Walking through Hell and hearing the screams, smelling the burning flesh and tasting the blood in the air, it doesn’t bring back anything – because it never went away. Just like Dean told Sam all those years ago, standing on a pier in Washington and trying to explain the unexplainable. Hell’s right here, in Dean’s head, and it always will be.
It’s there when he goes to sleep and when he wakes up, when he eats, when he talks to his brother, when he drives, when he fights, when he fucks, when he laughs and when he cries.
It doesn’t matter that this is the first time in years that Dean has set foot in Hell.
He never really left.