When Bobby asks him what he remembers, Dean's first response is: "Nothing." Partially because it's true. Blocking the past has been a survival skill for decades, and right now, with the passage back from Hell still fucking with his head, nothing's really clear. But the bits he's getting he doesn't want to make clearer.
Then instinct kicks in, decades of instinct that say you don't show weakness, and you don't open up anything because all the little soft pieces, whether that's the memory of a mistake or bone marrow, those pieces won't be pieces very long. They'll be cut and crisped into little knives that just move deeper until there's nothing soft left.
Dean knows. Lying, smiling, hiding is the only defense; hitting them before they hit you, always being ready to catch the knife some fuck—wearing a friend's face—throws at you.
So Dean adds to the lie, shores it up a little bit and smiles, because baring your teeth in one way or another is the best way to keep the weaker bastards off, the best way to get to their soft spots before they get to yours.
A second later, he remembers how he learned this lesson. How he taught this lesson. Just a little bit of a memory that he clamps down on hard, because, sickeningly, this is no longer Hell.
It's too good to be Hell, not enough pain and blood to be Hell. There's fucking sunshine, and candy bars and sore muscles and Bobby's standing there throwing holy water in his face, water which pisses him off but feels damn good at the same time.
If these people knew what he did...
Suddenly, Dean has soft spots again, and there they are, swinging knives at him, shocked and happy and unsure, just like he is. Bobby. Sam.
Dean can feel weak spots in his soul again, and they terrify him. So he goes back to the old lessons of lying, hiding, smiling, and he knows that they're not going to find out. They're not demons, they aren't looking, and Dean Winchester has hidden things so deep he's not sure he can find them again.
But still, they won't understand, and it scares him—few things do anymore—that Sam and Bobby will know. Because part of him is pretty sure that Hell is still not far away, no matter how good things are now. And if it comes back, like always, it will come with the face of a friend.