Mary once told Dean that every star in the sky was the soul of a person in heaven.
When Mary died and John became obsessed with finding her killer, becoming cold and distant, finding solace in drink and work, Dean found comfort in the stars. He'd stare up into the night sky and imagine his mom smiling down on him, telling him to be brave and watch out for little Sammy.
When Dean would find Sam curled up, miserable and teary-eyed, he'd hold Sam close and tell him the stories Mary had told so often, of angels and heaven and faith. As time went on, Sam took these tales to heart and became a true believer, while Dean slowly stopped believing that anything truly good could exist in the world except Sammy.
Sam escaped the life and went to college and Dean slowly became more and more like John, boozing and whoring and trying to forget that Sam left him. But Dean could never forget. How could he, when Sam was his whole life. Sometimes when the pain from Sams' betrayal became too much, he'd find a quiet place outside whatever crappy motel John had picked out- usually the hood of the impala- and stare at the stars, asking the empty sky why Sammy had to go and leave him.
Sam eventually got sucked back into hunting again after Jess died. Dean felt guilty for being so happy at the thought of his brother back when he was so distressed by Jess' death and Johns' absence. When Dean noticed Sam getting too tense (which happened about once a week or after a particularly tough hunt), he'd pull off the highway, hand Sam a beer and they'd lean on Deans beloved baby and look up in the night sky, not saying anything, just finding comfort in each other's silence.
It only seemed fitting that when Dean first took Sam it was under the starry constant of the night sky. The way Sam arched up under him in the faint light of the moon drove Dean wild as he thrust desperately into the giant spread wantonly on the hood of the impala, moaning and begging for more. After, they fell asleep, curled around each other in the backseat. The next morning they drove off like nothing had changed, but there was something between them now, something more, something neither of them had considered before but now couldn't live without.
When Dean was dragged sown to hell, Sam was lost. His only thought was getting his brother back, but nothing he tried worked. Hours after yet another failed attempt at trading his soul for Deans, he stumbled out of the bar and, beer still in hand, yelled at the cloudy sky, asking why, why Dean, before collapsing against the Impala, tears streaming down his face, begging for his brother back.
Just a few months later he got his wish, but then the angels came along and everything fell to pieces. Angels weren't the majestic, righteous creatures he had believed in, they were, as Dean rightly put it, dicks. The only exception seemed to be Cas, but it was soon apparent he greatly preferred Dean over Sam, perhaps even despised him, and Sam deserved it. He fucked a demon and drank her blood for power, he was pretty sure that didn't get him many brownie points with the warriors of heaven.
Somehow, through the apocalypse and Lucifer and Michael, the world falling apart around them, they still found time to sit and watch the stars. Watch the one thing that hadn't changed in their crazy, fucked up lives and think of the past and talk quietly of the future, as if they even had one. Or try to distract themselves as much as possible with roaming hands and rolling hips.
Dean dragged himself towards his brother, pain shooting through him with every movement, but he kept going, kept crawling through every agonizing inch to reach his brother. "Sammy" rasped every breath sending sharp shards of pain through his chest, and sighed in relief when he heard a soft reply. Sam was still here. For now anyway, he doubted even they could survive this time.
"Dean? We're not gunna make it are we." Sam breathed, unable to raise his voice above a whisper. Dean let his head drop to the floor and let out a shaky breath "I doubt it Sammy" he admitted, with a grim smile "No more running. No more fighting".
"Can you-" Sam paused, struggling to breathe for a moment "Could you tell me one of moms stories" he asked shakily "Just one more" Dean smiled softly and began, every word sending another wave of pain through his tortured body, but it was for Sam, it was worth it. He spoke softly, listening to Sams breathing growing shallower and shallower until it stopped altogether. Dean looked one last time at the familiar stars before he closed his eyes in despair and let himself slip away after his brother.
The soft sound of wings filled the air as Castiel appeared in the blood slicked wreckage of Singers Salvage Yard and slowly walked over to the boys' empty, bloody bodies and crouched down beside them. He gazed at them sadly for a moment before allowing a slow, small smile to appear as he leaned forward to brush the hair out of Sams pale face and affectionately place his hand briefly on the mark it had left on the righteous mans' arm, so many years before. He stood up and looked one last time at their still forms, they'd been fighting so long, so hard, they were finally getting the happiness and peace they deserved. He grinned broadly and looked up into the clear night sky as he disappeared and flew back to heaven. To the small roadhouse, where he was met with a firm slap on the back from Dean and a beer passed to him from a laughing Sam, Jess tucked under his arm. Cas looked around at the smiling faces around him, Mary and Bobby and Jo and Ellen, everyone they had lost, happy and content at long last and smiled, they had deserved this far too long.