Spoilers for "All Hell Breaks Loose" 1 & 2. Mentions of death.
Dean dies in the hospital, in his sleep, at age eighty-one, three years after his wife's fatal stroke. His kids are there, and their kids, and even the two great-grandkids, all keeping this final vigil together, because their dad taught them that there was nothing more important than family.
Sam is there too, but not as part of the family. He's the volunteer who offers to sit with Dean so the kids can take a break, can go get something to eat or run an errand or just to get outside for a few moments without the guilt of leaving Dean there alone, the one who awes the younger kids with his height and amazing stories, who always knows when Dean's visitors are tired or thirsty and anticipates their request, who somehow knows without asking that Robby prefers his sodas flat and doesn't mind drinking them hot, that little Sammy (who's a dignified, gray-haired grandfather nearly as tall as his uncle, and it's Sam, thanks) won't eat anything if it's got peanuts in it, that Carrie is allergic to chocolate and Maria can't have caffeine with her medication. Sam's a lifesaver to the Winchesters, supplying the touch of humanity and friendship and warmth long since gone from modern medicine, and when Dean's gone Carrie comes up to him, crying her heart out, and hugs him and tells him, choking the words out between sobs, that he should come to the service and he'll always be welcome at her house and she'd love to meet his family to tell them what a wonderful young man they've raised.
Sam leaves the hospital for the first time in weeks, finds a hotel, and cries himself to sleep, and it's not only because his brother is dead. It's because of the maternal way Carrie hugged him—Carrie, who'd been put into his arms when she was just four hours old, who'd worshipped her Uncle Sammy until the day he had to disappear from her life, and who now thinks he's some college kid who's just volunteering at a hospital to pad his résumé. Carrie, tiny little delicate baby Carrie, who spit up on his shirt every time he held her until she was seven months old; Carrie, who now thinks she's old enough to be his mother.
It's the first time his condition has slapped him in the face so hard, and it hurts.
The funeral is an ordeal. Sam slinks in after the service has begun, and smiles through his tears, listening to a eulogy that describes only Dean's settled life, after he shattered his leg and couldn't hunt anymore. There's nothing about Mom or Dad, nothing about their lonely, nomadic childhood, nothing about a yellow-eyed demon or deals at crossroads; there's only a fleeting mention of Sam, the lost uncle, told in the context of the stories Dean told his children. There's nothing about the misdemeanors and felonies, not a single mention of the murders that they pinned on Henriksen, clearing Dean's name, after a shapeshifter had taken the agent out. When it came to the end, none of that had had any place in Dean's real life. He kept it so much a secret that the entire family was baffled by his fierce insistence on cremation.
Sam sits there and listens, and his only regret is that he hadn't been able to play a bigger role in the life these people are remembering. He's gone before the service is over, back to the nomadic life that he's always hated and that now, ironically, is the only way for him to survive without someone catching on. Nestled beside the weapons in the trunk is a full set of the Highlander movies, a sarcastic gift from Dean when Sam left. They had come with a note, the apology Dean couldn't make himself say to Sam's face.
It wasn't Dean's fault, but he'd blamed himself anyway. Dean had been so determined to bring Sam back...
How could he have possibly known that a resurrection was permanent if you didn't request a natural life? That killing the dealmaker to free Dean meant making Sam immortal? It's not like demons offer to let you read the fine print.
Sam's seventy-seven, and he looks the same as he did on that night Jake shoved a knife into his back. There are no new scars, because every injury heals almost as soon as he gets it; bruises fade in a day, broken bones heal within the week. Fatal injuries knit themselves together in moments. The MacLeods have it better than he does, because they can die; he got beheaded once, and as soon as Dean assembled the parts for burning, the slash healed and he woke up.
Sam watches his nieces and nephews from a distance as the years drift by. He's the hero who rescues frail, white-haired Maria when a forgotten candle sets her house afire; he's the tutor who keeps Sam IV from flunking out of college English; he's the good Samaritan who stops when airheaded little Deanna, Dean's great-great-granddaughter, has a flat tire in a bad neighborhood. He's the shadow in the night who runs salt lines around their property and draws protective symbols where they can't be seen; he's the anonymous donor who leaves envelopes of hundreds in the kitchen when money gets tight, the total stranger who lists them as beneficiaries in wills and massive life insurance policies before he skillfully fakes another death; he's the last thing abusers and violent enemies see before they disappear forever, their bodies never to be found. A few times, when the loneliness gets too much, he's the orphaned college roommate, a lonely kid welcomed wholeheartedly into the warmth of a family who barely remembers their ancestor Dean but still practices his family-is-everything philosophy, and for a few years he knows that warmth again, before he sees his many-times-great-nephew and college roommate going gray and realizes it's time to leave again.
Sam's outlived everyone he ever knew. His world is as dead as the people he once loved. There's no going back. There's no heaven for him, not for a man who can't die; no hell, either, except the pain of walking away before his never-aging body betrays him.
He doesn't know how long he can guard them, how long before the course of nature means there's just too many of them for one man, even immortal, to keep track of. He doesn't know how long he can stand seeing those familiar hazel eyes in the faces of strangers. He doesn't know how many lifetimes a man can live as a hunter and protector before his mind begins to break down.
That doesn't matter. He made a promise, all those years ago, when Lianne put her foot down about her children knowing anything about hunting or the supernatural, a promise to Dean that if anything ever happened to him, Sam would be there to protect his family, and damn Lianne's squeamishness. He knows, deep in his heart, that Dean only meant that promise to bind him until the kids were grown, that it wasn't supposed to apply to later generations.
But if he can't die, he'll spend forever protecting Dean's family.
Dean taught him the importance of family, too.