"She'll be fine," Dean replies, thinking that Sam's desire to go back and let Polly know he's alive is a stupid one — of course. She's better off not knowing that Sam can't be killed, otherwise she, too, may wind up believing it's better for Sam to be dead, given that the supernatural is the only explanation for something like that. Plus, Dean truly believes that a little bit of horror in a kid's life is a good thing. It prepares them for their future — otherwise trying to survive in a Croat-infected world will be a shock when she's older and has to start being a productive member of her camp.

Dean shrugs, even though Sam can't see the motion through the seat. "I got my morals," Dean replies vaguely. "They ain't the same as yours, but I got 'em." He's not feeling this line of conversation, as it gets into territory that Dean isn't keen on exploring. It's bad enough that he's allowed Sam to become a liability in his life again — he doesn't need to admit to that weakness verbally.

Even though Sam has a point. He could have gotten himself killed. He may still get himself killed over Sam in the future. But they don't need to sit here and analyze that fact.

So he leaves it at that.