They need to be fine.
And they kind of can't be that.
It has been a long time since Sam acknowledged that they could have no happy ending. When there is a civilian or two they can't save on a hunt, it's just another part of their lives that's wrong, but Sam has fewer nightmares if he packs it into all the other wrong things like alcoholism, trust issues, abandonment and regrets.
It has been a long time since Sam stopped wondering why they put up with so much shit. He's taken on Dean's mantra: it's the job. It's the job. And it is, it is the job. Neither of them have answers as to why it's their job. Angels and demons have used to word 'destiny' so many times now, however, that Sam as an act of defiance refuses to believe any such thing. So all that's left is the random coincidences that make life a bitch, and Sam will just have to live with that.
It has been a long time since Sam accepted that they will never just die. Nothing can be that easy, not for them, and with how the currency of souls is high right now, heaven and hell are going to give it their all in harvesting each and every dead. Sam knows how to erase his footprints and hide from wendigos and avoid shadows, but regrettably he does not know how to get out of heaven.
It has been a long time since Sam has come to terms with the fact that Dean and him punch each other if they can't talk it out. Dean's hits hurt no less than those of strangers, but at least Dean is inherently justified as his brother. There are many things they do not talk about, things about each other, about the ones they've lost, but on some nights the tongue is loose and one of them spills a forbidden word like 'upset' or 'pause' or 'miss' and they'll be locked up in this place where they can't get out unless they speak or fight.
How long has it been since Dean has patted Sam on the back for no particular reason? How long has it been since Sam smiled easily at a waitress just for the small thrill? Since when did they stop counting their scars?
And now the world has come to an end, and they need to go out like heroes who did their job right. They need to go down swinging, like in all the good movies, with no remorse and cheeky grins. And they need to be fine in just that moment. And they can't.