Written in like 30 mins, so I'm sorry for the grammar mistakes you're bound to find. This is really sucky thb.
Sam's never been one to enjoy being alone.
When he was six and tiny, prone to waking up from dreams of scary monsters and the smell of fire, he'd sleep in the same bed as Dean, despite his dad insisting that he was too old for that sort of behavior. Motel beds were always too big for him, all hard springs and mildew-smelling. He'd wait until he was sure that John was out for the night and then crawl in beside Dean, comforted by the sound of his brother's even breaths.
He did that even when he was ten, older but not old enough to be embarrassed. He'd accidentally hit Dean with a bony elbow or knee occasionally, and Dean would stir from his sleep. "Ow, you baby. Go to sleep," he'd hiss, but he'd never make Sam leave.
When he was fifteen and they were expected to hold their own on hunts, he'd feel uneasy being separated from Dean or John. It was stupid and immature, and he never voiced his discomfort, but that didn't stop his chest from untightening only when they were done and back in the Impala.
Then he was nineteen and up to his ears with annoyance at John. Going to Stanford had seemed like a great idea at the time, and it was, to an extent, after he'd found Jess. She was his rock, his anchor for those few years. A face he could differentiate and trust from the blur of the seven thousand-something other undergrads.
Dean came back into his life then, and Sam had to get used to the crippling fear that came along with the job, the feeling of knowing that both of them could be killed by a witch or demon or whatever at any second. The feeling that he could be left alone if anything happened to Dean.
They'd both been hospitalized more times than he'd like to think about, but Sam can remember the time he and Dean had had a fight about something or the other and Dean had taken off to clear his head and Sam was angry and scared simultaneously. He'd given Dean a few hours and then caved, calling his cell over and over only for it to go to voicemail.
The local hospital called him the next day, informing him that Dean'd been found by some campers at the edge of a forest, ribs broken and several lacerations to his upper torso (courtesy of a werewolf, he found out later). His lung had collapsed and they'd been close to losing him twice during surgery. Sam remembered sitting next to his brother's still form for hours on end, leaving only when hospital staff made him. He remembers the debilitating fear of being left alone, of Dean leaving for good.
He lost Dean to purgatory and was lucky enough to find Amelia, someone who made him feel less alone. And then Dean was back and leaving Amelia wasn't as hard as it should've been.
Even at age 30, he'd wake up from nightmares and walk to Dean's room in the bunker; he'd never go inside but he'd stand outside the door and listen to the sounds of Dean sleeping.
But it's different now; he doesn't dream of Mary or Jess burning on the ceiling, or black eyes and Lucifer, or killing Kevin, but he jerks awake to the feel of his brother's head resting heavily on his collarbone, of the sick sound of Metaron's blade pushing through his brother's flesh and Dean's hand on his face as he whispers last words and then closes his eyes and falls against Sam.
He still shoves the cover back and walks down the hall to his brother's room and usually he gets halfway there until he remembers again and it hits him like a wall.
He's alone.