That explains a lot. Sam's already explained how Sam's world is lousy with demons, and they seem exactly like the witches of Dan's world in their cruel, self-serving machinations, in their eagerness to exploit people's sorrow and capitalize it to just make more hurt, more broken families, more graves.

And when he was with his sister, hunting a witch, she decided to do everything in her power to protect her little brother too. Dan sighs and rubs his face. He hasn't know Sam long, only a few months, and yet it's so obvious from what he's seeing here and the person Sam is that to lose Dean in such a way would rip Sam's heart and soul to shreds.

"Hunters really are all the same, ain't we?" His parents sold a soul to try and protect what they loved, and it backfired terribly. Dan wouldn't be shocked if Dean's deal had a similar dark end, even if he did in fact survive to see more Christmases. "You get into it because you think you got some kind of say in life or death, and then you just never get disabused of the notion."