In all his millennia of existence, Castiel had never met a pair of humans that burned as brightly as the Winchesters.
They were two brilliant spots of intrigue and adventure and familial love on his timeless life. Where one went, the other followed. When one fell, the other helped him up. You never simply met a Winchester. It was the Winchesters. And if, for some reason, you did come across just one, you could be sure the other was soon to follow.
Sam and Dean grew old doing what they did best – "saving people, hunting things," as Dean was so fond of saying. The family business. Even after joints grew creaky and senses not quite as sharp, the boys – men, Castiel supposed, but they'd always be young in his eyes – continued their work, watching each other more closely, covering each other's back when necessary (which became more often than not).
They never married, and Castiel supposed it was because they were married to each other in spirit. Any relationship that either Winchester shared with anyone besides his brother tended to get in the way of his familial tie. They certainly bickered like a wedded couple, picking at each other's faults and harassing one another for the smallest things. "Sam, how many damn times do I have to tell you not to leave my keys in the car?" "Dean, I swear to God, clean up after yourself or I'll beat you with that TV remote."
When the hair that Sam ran his hands through became grey and Dean's freckles hid under wrinkles born from laughing, they retired to Bobby's old junkyard, passing their days fixing cars and schooling the new generation of hunters. Castiel suspected they still took the odd case – Dean was always a restless one – but for the most part, he would find them seated in front of a fire or a TV, sharing moments of their past in silence.
That wasn't to say they didn't argue. They did, often and loudly. Their lives hadn't in any way been easy, especially concerning their bond. Dean had always been more attached to Sam than Sam to him, but Castiel saw that change over the years. When it became clear to Sam and Dean that the other was the only constant in his life, they took comfort in the fact rather than despairing.
It never ceased to amaze Castiel the strength of those two.
The Winchesters always seemed eternal, even in old age. But when Dean started to complain about his back more often and that leg of Sam's that never quite healed started acting up again, Castiel couldn't help but be reminded of the frailty and brevity of human life.
It was the darnedest of things when one of them finally broke. Dean had slipped in a puddle during one of their nostalgic walks through Lawrence. He'd landed badly and Castiel could still hear the crack of his hip against the ground. The trip to the hospital hadn't been especially rushed or frantic – after all, they were the Winchesters. They'd been through worse.
But Dean didn't get better. His hip wouldn't heal and he caught a flu that simply wouldn't go away. He was constantly feverish, rarely eating and hardly able to keep it down when he did. Castiel could tell long before the doctors or Sam could that Dean wasn't going to last much longer. He was an old man now and there was little they could do.
He went to visit his friend one night, pausing as he passed Sam in his customary seat next to the bed, sound asleep.
"Castiel."
The angel had looked up, unsurprised to find that Dean was awake. "Dean. How do you feel?"
Dean coughed a rasping laugh, shaking his head. "Man, I'm dying. Skip the formalities."
His smile had been too knowing, too accepting for it to be a joke. Castiel felt his shoulders slump incrementally. Of course. Dean had known he was dying.
"And I don't want you to do anything about it."
The angel nodded, expecting that as well. Dean was tired. He was tired of avoiding death and beating impossible odds. Death wasn't an end. It was rest.
That was the last time Castiel spoke with Dean. He watched, from a distance, as his health declined. Sam furiously did what he could, combing through both natural and supernatural remedies. Both Castiel and Dean knew there was no cure for old age, but they allowed Sam to perform hoodoo healing ceremonies and Enochian spells nonetheless.
When the doctors allowed Dean to go back home, Castiel knew Sam must have finally accepted that Dean wasn't going to make it. There was nothing medicine or magic could do for him, and it was best to allow him to pass the rest of his days in his own bed.
During the last few days, Sam rarely left his brother's side. They would spend hours at a time talking about everything or nothing at all, or simply sit in comfortable silence.
"Remember that song mom used to sing us, Sammy? Remember it?" Dean had whispered one evening, barely audible over the light storm outside.
"Yeah. I remember it," Sam replied, sensing that this was a turning point.
"Could you…could you sing it for me? Just once, I promise." Dean smiled weakly, and Castiel could see the light in his eyes flickering stubbornly, staying strong for just a little longer.
"Yeah, of cou– " Sam's voice caught, his hand combing through his brother's short grey hair. "Of course."
Carry on, my wayward son
There'll be peace when you are done
Lay your weary head to rest
Don't you cry no more
Dean's life left him in a satisfied sigh, a breath of, "Thank you, Sammy."
Castiel felt, for the first time, that he truly understood grief as he watched the younger Winchester's vision blur with tears, falling against his brother's face, a happy little smile curling his lips.
Dean was buried in the cemetery in Lawrence, a simple gravestone that was soon accompanied by another with the same last name on it. Castiel stood over the graves of his friends, humming a song that reminded him of adventure, and comfort, and resting in the arms of a lifelong companion.