Sam knew that there were things out there that could not be explained; powers that manifested that had no traceable origins. In fact, he knew better than most people, in some cases even more than Dean. But that understanding was not a universal understanding. There were some powers that were beyond even Sam Winchester's comprehension, such as the power of love and sacrifice, or the power of being an older brother.
He would never say it out loud, but he would never deny to himself that while Sam had many supernatural gifts, Dean had his own fair share of mysterious abilities that Sam envied.
For one, Sam didn't think it was possible to care so fiercely for someone that could also make him so unbelievable angry. For all the sorrow, pain, and guilt his big brother held in his heart, Dean carried a wide array of compassion and understanding, and he never once hesitated in the course of doing whatever was necessary for his baby brother or anyone else that he loved. Come hell or high water, Sam knew that above all things his brother had the ability to make him feel safe.
There was never any doubt that Sam's sense of security, whether true or false, always stemmed with how close he was in proximity to his brother. If they were surrounded by demons and all of hell's armies with death imminent, Sam would have still felt safe knowing he was with Dean. Sam knew that if anyone would always have his back, his side, be his support or be the one to go head first into whatever it was Sam needed to face, it would be Dean leading the charge for his little brother. If something happened that Dean could not save Sam from, then neither God nor all of His angels would be able to do any better than whatever Dean did for Sam.
If Sam was about to fall apart, Dean was all that existed that had any possibility of putting him back together. If Sam was in trouble, if he was angry, if he was sad, Dean had his own unique way of making Sam feel like everything was going to be fine, even if Dean had to face hell to make sure that would be the case.
He'd even done that on a few occasions.
For every fall, every spill, every wound, every tear, and everything that threatened to tear Sam apart, Dean was always the one he could depend on to make it better.
The day that stopped being true was the day he was no longer Sam.
So when the time came when Dean needed him, Sam was there. Call it a mix of brotherly love and a fear of being alone. Dean had been there for him as long as Sam could remember, and nothing struck a deeper fear in the man's heart than the thought of waking up one day in a Dean-less world. He'd done it a few times already, and it never got any easier. It was the one thing he hoped to never have to face again.
Dean was his big brother; an unstoppable force of cunning intellect, brute strength, ruthless rage, and in the worst cases, sheer determination fueled by the need to keep his baby brother safe at the expense of all else, including himself.
But losing Dean was not something Sam could ever wrap his head around. Not fully, even having already buried his brother once, who had always come back to him, the thought his brother being dead never once sunk in as a permanent possibility.
Until the nights he had to carry Dean to the impala, driving screaming down the road to the nearest hospital because his heart had stopped, or because his wounds were too much for Sam to know how to take care of properly. Or the nights he had to carry Dean into another rancid motel room, ranting and moaning about not wanting to go to a hospital again and that he was secretly fine – even when Sam knew he wasn't. But anything to make Dean happy, because that's what Dean would do for him.
Those nights Sam tended to wounds, carried him and supported him, even when Dean told him not to. True, his brother was proud, but he was still human, and like it or not Sam would tell him each time that it was their job to take care of each other, not just Dean taking care of Sam. Usually there was a gruff snort, a derisive sneer, or even a few snide comments after that argument, but Sam took them knowing that it meant Dean was still Dean, and he was willing to accept help if it made Sam 'happy'.
The nights Sam took care of Dean and he didn't fight it…those were the nights that made Sam scared. It was those nights when Dean was reluctantly vocal, or submissive to Sam's almost motherly doting over him; when Dean's eyes were just a little too glazed and looking a little too far off in the distance, filled with thoughts about things that Sam would never allow himself to consider.
The fact that Dean sometimes accepted his own death, or the prospect of dying, just a little too easily were the nights that made Sam fearful of waking up the next morning to find a body in the bed beside his own. He feared waking up not knowing whether his brother had called for him in the middle of the night, needing something only to go unanswered in his final moments of life.
Those were the nights Sam didn't sleep.
The first night Sam had truly been a little brother again was the night Dean had checked himself out of a hospital, out of the care of a doctor that had only given him weeks to live, to go back to a hole in the wall motel to be with Sam. Secretly Sam suspected that Dean didn't want to be alone, though had Sam been asked he was partly happy for Dean's company even given the circumstances. But knowing that his brother's heart would give out at any moment scared him into nights of sleeplessness, something Dean had berated him for.
He would finally try to sleep only out of fear that Dean's liberal utilization of guilt trips and nagging might strain his strength and do him in before Sam got the chance to save him, and he wouldn't have that.
On the nights they both slept, it was only after Dean had drifted off and Sam had snuck over to check on him one last time. He hadn't been prepared for the shortened breaths Dean would take in the night, the unconscious gasps or wheezes that would wake Sam in terror. He would never be used to how pale and clammy Dean's skin looked those nights, as if he were already gone and this husk was all that remained of the strongest man Sam had ever known.
Sam would sit on the bed and check his pulse, his temperature, grab water in case Dean needed it, or the few pills that the doctor had given him to help make his condition easier to bare. Then Sam would lie down and watch Dean while he slept, praying for his chest to just keep rising and falling through the night. One more day and they would find a solution. He wouldn't have to watch Dean wither away another minute.
Sam would sit there for minutes, or maybe even hours. Sometimes his thoughts would be racing, and other times he would just meditate in the quiet. The first night, Sam sat at the edge of the bed, watching Dean in his own fitful sleep. Then he drifted to sleep, more tired than he realized for all the fretting and the waking nightmares he had of a dead brother. In the morning he would be curled into Dean's back, his head pressed just slightly his shoulder blades, feeling the cool dampness of his skin and the sporadic beats of his heart.
Most nights where Dean's injuries were this severe, this was usually how Sam fell asleep. Even as an adult, Sam felt that being near his brother would somehow keep Dean safer in those tenuous moments of not being sure whether he would be okay or not. If Dean needed to reach out or gasped for him, Sam would be there just within arm's reach, and it was worth all the comments Sam had thought he knew Dean would throw at him in the morning whenever he woke to find his brother curled up next to him in bed.
Except those comments never came.
At first Sam was confused. He had expected to be berated and chastised for acting like a child, or jokes about incest to lighten the mood, yet none of those ever came. Dean would usually just lie there, still, doing his best not to wake his brother despite the bouts of pain, knowing that Sam needed the sleep just as much as he did. Sam never said anything either after those nights, but the more he thought about it the more he thought he might have pieced it together.
It seemed that Dean understood why Sam snuck into his bed on nights like this.
As children, Sam had always tiptoed into Dean's bed whenever he was scared. As young boys who knew all too well that there really were things that went bump in the night, Sam coming to Dean happened fairly often. But for all the things that Dean chided his brother for, sneaking into his brother's bed in the middle of the night, be it for protection or comfort or both, had never once been among them.
Sam had not expected it to remain that way as adults of course. He was a man now should be acting like it, especially in times when his brother needed his strength. Not that that was ever an issue. Dean hobbling to and fro was never first offered an arm to lean on. Sometimes that arm found its way around his shoulders instead; gently guiding him with support should he ever fall. And sometimes Dean would brush that arm off, but sometimes - sometimes he wouldn't too. Maybe it was just to make Sam feel better, so he felt like he was actually doing something for his big brother. Or maybe, just maybe, Dean really needed to be supported during those times. Whatever it was, Sam made sure to be there. But he also needed to make sure Dean was okay for his own sake, because the next time Sam woke up curled into Dean's back – ears' straining to catch breathing that wasn't his own – he knew that it was fear that still drove him to his brother's side. Only this fear was not based in supernatural phenomenon. No monsters or ghouls or moving shadows pushed him to his brother in the middle of the night. It was the fear that if he wasn't close by, his brother might let go.
It was the fear that Dean might leave him behind and go somewhere that Sam could not follow.
It was the fear of a dozen birthdays and Christmases passing with no one to spend them with.
It was the fear of every family reunion from this point on taking place in a cemetery.
It was the fear of spending countless nights feeling a familiar presence, but turning to find out that there was no one there.
It was the fear of being along and not knowing how to be.
It was the fear of burying the last thing in the world that will ever love you unconditionally.
And Dean never said a word. For all the things he got on Sam's case about, he stayed mute on this one because Sam knew that deep down Dean shared every one of his fears on that matter. So he would allow Sam to take up half his bed and almost all of the covers if it means his eyes weren't as swollen in the morning from crying. He would allow Sam to snore gently into his shirt at night if it meant he slept a little easier, even if Dean didn't. He would let Sam do all these things and more if it made him feel better, because both brothers knew that whatever comfort they found in each other might be the last they ever find in this world, and they could not afford to lose that even as they faced losing each other.