They've been driving for over an hour in a sort of frozen silence when Dean snaps.
"This isn't who you are, Sammy."
Sam looks at him with a kind of incredulous glare. "Not who I- Dean, how the hell would you know? You've been gone; you were in hell! You think I'm gonna be the same person I was before?"
Dean stares at him for a good thirty seconds before remembering he's driving; he makes a mental note to pull into the next motel he sees. When he finds his voice again, it comes out low and gravelly.
"You can't pull that shit with me, Sam."
Sam glances over at him, confused. He barely has time to get out a "What?" before Dean swings sharply onto the side of the road, throwing Sam into his door.
"Sammy, I know you. Jesus, I've pulled bullets out of your shoulder, I've hunted beside you, I've-" he swallows, and carries on. "I've fucked you, Sam; you don't get any fucking closer than you and me. So don't you dare sit there and tell me I don't know you! Four months in Hell doesn't cancel out nearly thirty years of... of us. I know you, Sam, in a way no one else ever could. And I don't... I can't lose you again."
Sam's jaw is hanging open; he snaps it shut and reaches over to clasp his brother's shoulder. "Hey. Hey. I'm still here, Dean; I'm not going anywhere. Okay?"
Dean nods carefully. "Yeah, Sammy, I know."
Sam kisses him swiftly and grins, relieved. "Okay. What do you say we find a motel or something? Come on."
Dean starts up the car again, and pulls out onto the highway. "Still, Sam... this isn't who you want to be. Right?"
His brother swallows hard before replying. "No, it's not who I want to be. But what if... maybe it's who I have to be."
And there's nothing Dean can say to that - so he drives on in silence.