It has been a long time coming, in hindsight. If Cas thinks hard, he can remember the first time it became obvious. He can easily recall the first time Dean had to sit out a hunt, the first time a creature got away because Dean was too tired to chase it, and it should have been obvious.
He remembers when the lines first started to form on Dean's face, knows them all, has learnt them by heart with his fingers in the afterglow. They have always been a sign of something darker, Cas knows. They've always been a ticking clock, a reminder of Dean's mortality compared to Cas' eternal sentence, but Cas has been able to push it to the back of his mind until now.
Even when Dean had to give up hunting, even when Cas started to be mistaken for younger relative when they bought groceries, even when his hair was more grey than brown and more white than grey, Cas has been able to ignore it, focus on the past and the present and pretend that the future wasn't inevitable. But of course, it is.
Dean looks sad. Cas reaches under the covers and holds his hand, traces the veins, prominent as they now are, and thinks of how they carry lifeblood to Dean's heart. In a few days, perhaps, they won't. Dean squeezes his hand and Cas thinks how weak he feels.
"I guess I just didn't think that after all we've been through, after all the monsters we've fought and all the demons we've put underground, that it would end like this," says Dean, smiling a little sadly.
Cas swallows hard, biting back tears he hadn't known could come. This is the Righteous Man. This is his Righteous Man, and he should not be dying. Not like this. Not even at all.
"I could prolong it," Cas says, thickly. "I could - "
He reaches out to touch Dean, but Dean recoils.
"Don't," he replies, and Cas withdraws his hand, the hurt from the rejection almost as bad as the fear of the inevitable. Dean sighs. "I don't want you to do that, Cas. I don't want to carry on any longer than I have to." He gestures down at himself, at his betraying body hidden under the thick comforter. "Not if I'm like this. It's no life, Cas."
Cas nods. He knows that Dean isn't leaving him, but it's hard to think otherwise. It still feels like a personal slight that he was destined to love someone who could not last forever.
"I had a good run, though," says Dean, and he's smiling a little less sadly now, his eyes crinkling. They're still the brilliant green they always were, Cas thinks, swimming with good humour and kept promises, and it offers some small comfort that even with the decay of his body, Dean's mind is still untouched. "Saw some things, did some stuff."
"Yes," Cas agrees. "More than most."
Too much, he thinks. If Dean had exerted himself less, perhaps he could have lasted longer. Perhaps he couldn't. Mortality is fragile and fickle, a small bone in a greater body.
Dean looks at him, swallows hard the way he always does before he's about to say something difficult.
"I do love you, you know," he says.
Cas wonders why, after over forty years, Dean still finds that hard to say. Cas knows he loves him. It's written all across his soul and he thinks Dean would write it across the sky if he knew how.
"I know," Cas replies, and he squeezes his hand again in an attempt at reassurance. Dean clearly appreciates it because his smile widens and he leans over to kiss Cas.
He still tastes the same, Cas thinks, and that makes him happy for a brief moment. They still fit together as they always have done. Dean is weaker but he still knows how to do this.
Dean pulls away, and closes his eyes. He's only going to sleep, Cas knows. This isn't the end. Not yet.
"See you on the other side," says Dean.
He doesn't. He sleeps, and Cas lives on, and perhaps Dean does too.