Disclaimer: Nothing's mine...
Dean can't help but laugh.
Really. How ironic. The one who Dean wanted to save but who didn't want to be, the one who said they weren't family anymore wants to save him ? Because they're brothers ?
That is so idiotic and so absurd and so incredibly sad that Dean can't help but laugh.
He's sad and disappointed and most of all, angry.
But while before he would have just buried everything under his though-guy act, now it's different.
Dean is different.
The Mark of Cain has made him different.
Now he will have his vengeance.