Set during Season 2 Episode 11, "Playthings". The first bit is the exact dialogue from the show, up until the kiss. I wanted to give a bit of context.

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Dean marched purposefully towards their room. He saw from down the hall that the door was slightly ajar, the key dangling from the lock. He shook his head as he snatched the key, swinging the door shut behind him. He glanced over at Sam, who was sitting in a chair near the door. "There's been another one. Some guy just hung himself in his room." He began digging around in his bag.

"Yeah, I saw," Sam mumbled.

"We gotta figure this out and fast. What'd you find out about granny?"

There was a pause before Sam spoke. "You're bossy."

Dean turned around slowly, examining his brother. "What?"

Sam threw his hands in the air. "You're bossy." He giggled. "You're short."

Dean's eyebrows knitted together. "Are you drunk?"

"Yeah. So?" Dean looked away and scrubbed a hand over his face. He heard Sam mutter, "Stupid."

He turned to face Sam again. "Dude, what are you thinking? We're working a case." He stared pointedly at Sam, waiting for an answer.

Sam's eyes were glazed and unfocused, sweeping listlessly across the floor in front of his feet. "That guy who hung himself… I couldn't save him."

Dean shook his head. "What are you talking about? You didn't know. You couldn't have done anything."

"That's an excuse, Dean," he spat. "I should have found a way to save him. I should have saved Ava, too."

"Yeah, well, you can't save everyone. Even you said that." Dean started to turn back to his bag when Sam pounded his fist on the table, hard.

"No, Dean, you don't understand, alright? The more people I save, the more I can change."

Dean squinted at Sam, trying to figure out what he was on about. "Change what?"

"My destiny, Dean!"

Dean rolled his eyes. "Alright, time for bed. Come on, Sasquatch." He approached the chair and wedged his arms under Sam's, hauling him to his feet. "C'mon." He began walking Sam towards his bed.

"I need you to watch out for me."

"Yeah, yeah, I always do," Dean mumbled.

"No, nonono," Sam slurred, pushing at Dean's hands and turning to face him. "You have to watch out for me, alright?" He steadied himself on Dean's arm, making eye contact as best he could while the room spun around him. "And if I ever turn into something that I'm not… you have to kill me."

"Sam…"

"Dean, Dad told you to do it. You have to." He gripped Dean's arm tightly, willing him to understand.

Dean stared incredulously."Yeah, well, Dad's an ass. He never should have said anything. I mean, you don't do that! You don't lay that kind of crap on your kids!"

"No. He was right to say it. Who knows what I might become?" Sam gestured uselessly. "Even now, everyone around me dies!"

Dean took a second to think, trying to figure out how to end this conversation quickly. "Well, I'm not dying, okay? And neither are you. C'mon, sit down." He pushed Sam down onto the bed and tried to turn away, but Sam held onto his arms.

"No! Please, Dean, you're the only one who can do it. Promise."

Dean shook his head. "You can't ask that of me."

"Dean, please. You have to promise me."

Dean stared helplessly at Sam. He couldn't promise that. But it was what Sam needed to hear in that moment. It's fine. He just needs to sleep it off. He won't even remember this tomorrow. "I promise."

"Thanks." Sam gripped Dean's face in both hands. "Thank you." Before Dean could wrap his head around what was happening, Sam was kissing him. And Dean was kissing him back. He grabbed a fistful of Sam's shirt and leaned into him. He felt Sam's hands on his back, pulling him down onto the bed, and quickly stepped back. He looked down at Sam, wide eyed and flushed, surely a mirror image of his own face.

"What?" Sam asked quietly. Dean didn't have an answer. Everything in him wanted to push Sam down onto the bed and fuck him senseless, something he hadn't done in years, not that he hadn't been thinking about it. All the tension that had built up since they had started hunting together again was exploding out of his veins at once. All the assumptions by strangers that they were a couple, all the touches that lasted half a second too long, all the carefully averted eyes when they changed or showered, all the late nights in motels spent jacking off when they didn't know for sure whether the other was awake.

When Sam had left for Stanford, Dean hadn't known for sure if he would ever see his brother again. And he had assumed that if he did, their physical relationship would be over. As the months rolled by, he began to brush it off as just something they did as kids. It didn't mean anything. It was wrong. It was even more wrong to pick it up again as full blown adults, people who should be mature enough and have a strong enough moral compass to avoid this sort of thing. He knew he should probably say some of this to Sam, tell him he was just drunk and would regret it later. But all that came out was, "Are you sure?"

Sam nodded and barely had time to pull off his shirt before Dean was on top of him, his mouth on Sam's skin. Dean left a trail of hickeys on his neck, feeling like a damn teenager but too desperate to get embarrassed about it. He worked his way down Sam's torso, licking and biting, running his hands all over Sam's skin. It felt foreign and familiar all at once. Some things were the same, but Sam was also older, larger; old hunting scars peppered his skin alongside newer ones from the past two years. Dean paused at Sam's waistband and glanced up, seeing Sam's head tilted back, eyes closed, mouth slightly open, fists clenching the sheets. He undid Sam's fly and stood to yank pants and boxers off in one swift movement, then dug in his pockets for lube before undressing himself.

Dean climbed back onto the bed and wasted no time spreading lube over his fingers and sliding them into Sam. Sam groaned loudly and Dean resisted the urge to cover his mouth, an instinct born from years of making each other cum with John just a few feet and a thin wall away. He leaned up and kissed Sam forcefully, winding one hand into his long hair, relishing the feeling of Sam moaning into his mouth. He pressed his body into Sam's and slid their cocks together, precum already slick against their skin. Dean pulled away and kneeled between Sam's legs. He licked a long, slow stripe along the underside of Sam's cock before taking him fully into his mouth with one smooth movement. Sam whispered Fuck and bucked his hips, his fingers clawing at Dean's scalp and shoulders. Dean was pleased to find that, even after all those years, he still knew exactly how to make Sam come undone. His head bobbed rhythmically, matching the pace of his fingers in Sam's ass. He brought Sam right to the edge, shaking and panting beneath him, and pulled away. He sat back on his heels and watched Sam catch his breath, waiting.

Sam slowly opened his eyes and looked up at Dean, his pupils blown wide, eyes glassy with lust and alcohol. "Fuck me," he said quietly. Dean smirked and retrieved the discarded packet of lube from the bed, using what was left to slick up his cock. He took a few seconds to stroke himself, watching Sam watching him, before climbing on top of Sam and sinking into him. He tried to go slow, to savor this in case it never happened again, but he couldn't help himself and soon he was slamming into Sam with all his might. His skin stung where Sam raked his nails across hard enough to draw blood. Sam dragged his lips across Dean's jaw to his ear, nipping at the soft skin, and Dean let out a strangled Jesus Christ, Sammy before leaning down to flick his tongue over Sam's nipple. He squeezed a hand between them and picked up a hasty rhythm on Sam's dick, listening to his brother getting progressively louder. He always was a fucking loud mouth, he thought. He was sure anyone and everyone in the hotel had an idea of what was going on, but he didn't care.

Sam came with a shout and an incoherent stream of profanities. His back arched off the bed and then fell with a muffled thud. Dean buried his face in the crook of Sam's neck and began chasing his own orgasm, thrusting erratically. He let himself be overwhelmed by the sound of Sam's tired moans, the smell of salt on his skin, the creaking of the bed springs beneath them. He bit down hard on Sam's shoulder as he came, then collapsed heavily on top of him. They lay completely still for a few moments, breathing like they had just run a marathon. Dean finally mustered up the energy to roll himself off of Sam. He closed his eyes, his head swimming. He felt Sam lay his head on his chest and curl against his side. Dean wrapped his arm around Sam and held him firmly, batting away the feelings of guilt and frustration trying to creep into his brain.

"Thanks," Sam said again. Dean nodded idly and ran his fingers along Sam's spine, appreciating the smooth skin under a thin sheen of sweat. He pulled a blanket over their bare bodies and allowed himself to be lulled to sleep by the gentle rise and fall of his brother's breaths.