Dean's face paled a little when Sam said that it was his second time having a flashback... this was bad. If sam was starting to have flashbacks of what was behind the wall in his mind, it must have meant that there was a crack in it, right? That's how walls worked, and small cracks led to demolished walls. There was a reason Death put the wall there in the first place, whatever was behind it needed to stay away from the rest of Sam's brain, and apparently that wasn't working too well.
Great.
"Listen, you never let me get turned, and even if you did, you weren't you, Sammy. You gotta stop thinkin' about this." Dean let his grip loosen only when he was sure Sam was alright to stand on his own. "I know it's probably easier said than done but you can't keep poking at it if you want the wall to stay up. We shouldn't even be talking about it!" There was concern in his voice, and despite his own words he was still watching Sam carefully to make sure everything was alright, for now. "...We gotta talk to someone, see if anyone can help you with this. I don't want to think about what's going to happen if this thing comes down." Dean had a hard enough time thinking about his own time in hell, he couldn't imagine what Sam would have gone through sitting in a cage with Lucifer.