Sam immediately is emotional as he listens – in part because his brother is opening up and talking to him, like he asked, and in part because he has so much empathy for how Dean is feeling and what he’s been through. Not sympathy, but empathy. Because Sam KNOWS.
Sam: Dean, you did what you had to do.
He’s right. Dean said yes to Michael to save Sam, to save Jack, to keep Lucifer from taking over our world. I don’t think he regrets that, but what has happened since, Dean feels is on his shoulders.
Sam’s empathy must really get through to Dean, because he voluntarily shares more with Sam about how the experience of being possessed felt and how it impacted him.
Dean: And it wasn’t a blank. I don’t remember most of what Michael did with me, because… I was underwater, drowning. And that I remember, I remember every second of it. Fighting, clawing for air… I thought I could make it out, but I… I wasn’t strong enough. And now he’s out there hurting people. And it’s all on me, man. I said yes. It’s my fault.
The camera pans over to Sam, and his eyes are brimming with tears, his expression etched in pain.