"And I'll tell you another thing: If you screwed up my car, I'll kill you."
Even as the words come out of his mouth, Dean hates himself for saying them. It's not that he's said something wrong, because Sammy laughs, that "I can't believe my brother's such an idiot" laugh he remembers from when they were kids, when he'd start making wisecracks about the spirits they were hunting to keep them both from being scared. It takes him back to a simpler, happier time.
He hates himself because it wasn't what he wanted to say. He has so many words and phrases spinning around in his head, so many things he would like to tell Sammy right now. Things like, "That took guts, driving her in through the wall like that. I'm not sure I would ever have thought of it. I'm really proud of you."
Or, "Man, it's been good to have you by my side again. I've missed you so much."
Or maybe, "When I saw you in the car with that witch on top of you, I just about went ballistic. I'm surprised I didn't do something stupider than try to shoot the ghost in the head."
Or even just, "Those burns on your chest—they look pretty bad. Are you sure you're okay?"
The thing is, there was a time when he might have been able to say those things. Not that he's ever been good at sharing his feelings—he's always had a horror of "chick flick moments"—but he used to be able to talk to Sammy. They used to be close.
Now, though, he's not even sure he knows Sam anymore. Some things haven't changed: Sam's still an out-of-the-box problem solver; he still somehow remembers everything Dad taught him about hunting; he still disapproves of Dean's eating habits; he still sleeps on his back with his arms crossed over his stomach or one hand curled up in that cute little way by his head.
But other things… other things are different. He's got so much anger and resentment toward Dad, and maybe toward him, too. He's given up on finding Mom's killer, given up on her altogether. The way he talked about her—it still angers Dean. He's glad he was able to stop Sam's apology earlier, because he's not sure he's ready to forgive him for that yet. And then—perhaps the ultimate source of the disconnect between them—Sam's determined to be normal, to have a normal job and a normal wife and normal kids and a normal little house in a normal little suburb.
He's on a path that Dean can't follow and wouldn't want to if he could.
So with all that going on, maybe it's best to keep it impersonal. And that, he knows, is why he settles on the one sentence floating around in his head that isn't sentimental, that doesn't make him vulnerable in any way:
"If you screwed up my car, I'll kill you."
And Sammy laughs. For now, that'll have to be good enough.