Dean did this thing when he was feeling sick.
For as long as Sam could remember, Dean had always done it. When Dean was nine and fell in Bobby's yard and had somehow managed to rupture his appendix, and the hospital took the organ out and sent him home a day later to make rooom and he was sick to his stomach? Dean had crawled onto Bobby's lap and curled into a ball. Or when Dean was fifteen and got a partuicularly nasty strain of flu and just couldn't keep warm? He'd just pulled on one of his thicker hoodies and curled up on Dad's lap.
"Dean's not feeling too well, Sammy." their father had offered as an explanation at the time, "He's not really aware of what he's doing."
Dean, despite his common rants about being the manliest motherfucker in the world, was a cuddler. He had cuddled with Sam when they were younger, he had cuddled with the stuffed moose Sam had managed to smuggle into Dean's sleeping arms a few years ago on the older man's birthday, so Sam wasn't too concerned when one day, after the battle with Metatron, Dean had wrapped himself like a giant, human burrito, and climbed onto Sam's lap.
Dean had been honest with Sam from the moment he'd woken up. He had told Sam all about how the First Blade had felt in his hands, how the Mark of Cain had him wanting to tear everything to shreds and bathe in their blood from the very beginning, how knowing Sam was gonna hate him for what he'd let himself become.
"I'm a fuckin' demon, Sammy!" Dean had manage to say without hurling, "An' I don't know how we're gonna fix this!"
Yeah, then Dean had puked.
Sam had merely dropped down to his knees next to his older brother and held him tight, whispering that everything was okay, that he knew it was the Mark and the First Blade from the very beginning, that Dean could suddenly sprout bunny ears and a clown face and Sam wouldn't mind because Dean was alive (more or less) and he was here and Sam wasn't going to go anywhere because they both needed to be together to figure this out.
So when his big brother had gone limp in his arms and mumbled "stupid sasquatch", Sam had known the were going to be fine.
At least, that's what Sam had thought before Dean got sick.
Sam, worried about the big brother he'd just got back, had summoned Crowley. While the taller man had rambled on about Dean being sick, throwing up blood, his nose hemorraging like a mother, how he could barely speak he was in so much pain, the King of Hell just rolled his eyes.
"He just 'woke up' a few days ago, Moose." Crowley snorted, like any of this was funny, "He's just adjusting from human to demon. He's gonna feel like right shite for a while."
Apparently all Sam could do was make Dean comfortable. He brought his big brother water and soup, which always came back up. He went out and bought Car & Driver magazines and even an issue of Good Housekeeping that was doing a special on Apple Pie. Sam got wet washclothes and cleaned his brother off the one time Dean hadn't managed to make it to the bathroom. He kept his brother warm, hydrated, and entertained for two days before Dean had snapped.
"Will you just back the fuck off, Sam?" his older brother had snarled, eyes flashing black, "Don't need you to fuckin' baby me!"
Sam had flinched, apologized, and backed out of his brother's bedroom in ten seconds flat. He hadn't meant ot upset the older man. Sam, bless his heart, was hovering because a part of him couldn't believe this was all real, like a twisted dream. He felt that if he blinked, he'd wake up and find his brother's body in his room, bloody, pale, and dead.
Swallowing the bile that forced its way up at the thought of Dean dead and gone, Sam shuffled around the Bunker, straightening, cleaning, just trying to distract himself. With a weary sigh, he sat on a couch in the library and stared off into space. The next thing he knew, he had a lap full of Dean and a murmured "sorry S'mmy", which was fine with Sam. This meant Dean wasn't really mad at him, just frustrated with himself.
As Dean drifted off to sleep, Sam wrapped his arms around Dean and held onto him. He pushed his face into the dark blond hair and thought about the future. What if Dean didn't make it through the conversion? What if Dean was only doing this because as soon as he was better, he was leaving? What if he made Sam leave? He just got his brother back, dammit, he didn't want be separated anymore!
"S'mmy?"
Dean's head popped out of the blankets to look up at his distressed baby brother, "Whassamatter?"
Sam rubbed his face on Dean's blanket covered shoulder, "Nothin'. Just thinkin'."
"Well, stop. I can hear the gears turning from here." Dean mumbled, burrowing (he didn't snuggle dammit!) back down into his blanket cucoon, "Gonna burn your brain out."
So Sam stopped thinking about the what if's that bothered him. For now, Dean was stuck with him and if Dean chose to run off, well, it's not like he could take the Impala. It was demon proof. Which got Sam thinking again...
"Hey, Dean?"
"Hmm?" Dean mumbled, opening a single bleary eye.
"...how are you gonna drive the Impala if it's demon proofed?"
Dean blinked and then jumped off Sam's lap, taking of at a run. Sam blanched at his brother and followed after him. As he caught up with him, Sam couldn't help but smile at the sight of the older man stomping his foot like a girl, cursing up a storm as he tried to get into the trunk.
He had gotten the lid open, which Sam thought was some kind of miracle, but for the life of him, Dean couldn't get his hand to go inside. It kept stopping short of the inside. Growling, eyes flashing black to green to black again, Dean slammed the trunk, and turned to glare at Sam.
"This what was botherin' you?"
Sam shuffled in place, "Well...um...I was thinkin' about what you were gonna do when you're better...I was thinkin' that you might try to take off or something...and, well, the Impala..."
"...is demon proof..." Dean muttered, eyes flipping to molten green, "Can we go back inside now? Dunno about you, but m'kinda tired. Tryin' to get past a freaking Devil's trap in my own freakin' Baby is exhausting."
Sam simply smiled and led Dean back inside, where, instead of going back to the couch, went to Dean's room and climbed in bed with him, "Can I stay here with you?"
"'Course, Sammy." Dean yawned before curling up under the covers and into Sam's side, "S'good thing you've always been a friggin' furnace, Sam, 'cause I'm freezing."
So, Dean did this thing when he was sick. He cuddled (though to say to his face would mean a swift punishment) into the nearest source of safe heat. And Sam? Sam didn't mind one bit because when Dean did his thing, even now when the older man was a demon, it was still his big brother. Dean wanted to be close to Sam, despite all the crap they'd been through months before, and Sam could actually do something to take care of Dean, something that had only happened a handful of times since Sam could remember.
It was the best of both worlds.