Because, as much as a lot of us joke, Dean would be devastated to lose Cas, especially by his own hand. But he'd obviously never choose him over Sammy either.

And Sam, he probably wouldn't even expect that decision. Sam is impure, he's let Dean down, he's never been strong enough. And Castiel is an angel. Even now, even fallen, he is so much more than Sam could ever be. Not to mention their profound bond. To him it's logical, and when he lives and Cas dies it's hard not to feel guilty and like it could of been different if Dean wasn't his brother.

And Castiel? He'd know immediately he's not getting chosen. And he accepts that. He never had anything on their bond. As much as it breaks his heart, its something he has always know he would have to accept one day.

Thank youuuuu!!! :D Ahh I'm always super happy when I hear from fello multishippers! Concerning your question, I hope this is answer enough:

Malcolm/Reese/Francis is the answer to Wincestial.