Dean isn't stupid, contrary to what people seem to think. If there is anything Dean knows, it is his brother Sam. He knows how to deal Sam when he is sulking, angry, or upset, so when Sam starts avoiding eye contact and somehow manages to both cling closer to Dean and pull away at the same time? He knows that something is amiss. His big brother senses are going off, body tingling with the knowledge that something is wrong with the only person who has ever really mattered, and there is no way that is going to stand with Dean. The next weeks are spent worrying, trying to uncover the sudden reason for Sam's sadness. Dean's job is to take care of his Sammy, and if someone or something was interfering with that, The older boy needed to take care of it.
Sam is freshly 14, still scrawny, all bones, sinew, and wiry muscle. Old enough to stretch for independence, but young enough to cling to Dean during thunderstorms and scary movies. The backwater town swelters in the May sunshine, leaving the boys shirts sticking to their chest in the humid South Carolina heat. They're in town investigating a couple of deaths, John had even promised that Dean and Sam could stay in town until the end of the school year before they headed up to Bobby's place. The actual promise didn't mean much, he was likely to go back on his word without a second thought, but Dean would cross that bridge when he came to it.
Dean has always been it for Sam, the only person who ever tried to understand him. In a life where the hunt is all that matters, and the role of a father has been placed on the shoulders of a willing and capable Dean, the two boys had learned to make do with themselves. Their careful arrangements and systems only briefly disturbed by their father's abrupt comings and goings. John Winchester had never been Sam's true father, nor had he been Dean's. John had hunted and lived under the illusion that he had Dean wrapped around his little finger. In reality, Dean allowed their father to think this because that was simply easier for both him and Sam. John hunts, and Dean and Sam able to carve out some semblance of normality in the life they live.
It starts with the little things. During the day Sam will shy away from Dean's touch, hiding in the shadows away from their father's piercing gaze (the hiding isn't new, but usually Sam takes to folding himself into Dean's side in an attempt to merge into his side. These days he spends time curled under the half clean sheets on his side of the bed with his back towards his big brother). Sam was never one for vanity, but he begins spending longer and longer in the dirty motel bathrooms, only leaving with extensive cursing and shouting on Dean's part. Even Sam's clothing habits change. The boys had never had a lot of money to spend on clothes, so, in general, Sam ended up in a lot of Dean's hand me downs. As it happens, these days Sam has been folding himself into Dean's bigger sweatshirts, hiding behind large t-shirts and pajama pants. But really, the biggest change was Sam's food intake. Sam was never a big food eater, not like Dean. He preferred side salads and apples for breakfast, but it was not like he ever didn't eat. Currently, Sam had taken to picking at his food while his father and brother were too busy scarfing down their own meals to notice until he was excused. Dean realizes it one day as he is watching Sam stare at his own reflection in the mirror, and really he's an ass for not figuring it out earlier.
None of the Winchesters had ever had particularly high self-esteem, sure Dean liked to act like a self-righteous bastard, but, in reality, he has issues of his own. However, his are nowhere near the size of his Sammy's self-esteem struggle. Because even if Dean tells Sam that he loves him every day, tousles his hair constantly, and hugs him constantly, the real heartache comes from Sam's actual father. John Winchester is not a puppies and encouragement kind of guy, that man was lost when he watched the love of his life burn on the ceiling. Instead, he preferred to point out flaws and threaten his children into submission. He would leave the encouragement to his oldest son. The important thing was to keep moving forward, to keep closing in on yellow eyes, regardless of pain or fatigue. Leave Dean to deal with temper tantrums, no harm was done, Dean preferred to have full reign over his little brother anyway.
Really, it was no surprise that Sam was getting the worse of Winchester self-esteem issues. He was the baby of the family and he had yet to hit his growth spurt, leaving the butt of all jokes. The thing was, Dean's jokes were accompanied by a grin or an affectionate noogy. Dean made it a mission to make it clear to Sam that the jokes were all made in jest. Their father, however, said things that sounded like jokes, things that should have been jokes, but were made with straight faces, no humor coloring his tone or the words that left his mouth. No child should ever have to hear the words waste or useless used against them by their own father, no matter how drunk that man may be at the time. In the end, Dean isn't always around to buffer, and sometimes the words are spit even when he is there. ( and dammit, the thought of that shouldn't break Dean's heart.)
Dean doesn't do chick flicks(at least, that is what he tells himself) however there are times where he is required to put his ego on the line in order to take care of Sam better. So one night when he catches Sam staring at himself in he mirror, instead of cracking a joke he presses Sam to his side and pulls him into a hug. "Sammy." It's not a question, and he doesn't say anything else, just a whisper of his baby brother's name, muttered against the dark mop he likes to call hair. It's not okay Dean supposes, this is clear by the way that Sam stiffens before utterly melting into the embrace, fingers coming to grip Dean's shirt like it's the only thing keeping him tethered to the earth. The whole thing hurts Dean in a place he thought was safe from hurting (It's the part of him that shatters completely when Sam leaves him to step into the world via Stanford, but that's years, lifetimes even, away from this moment). After a few minutes he pulls away and grins wetly down at Sam, Dad's gone for the night, how about a pizza and a movie kiddo? The smile he gets in return is hesitant and if Sam hasn't regressed back to the age of 3 then Dean hasn't raised the kid since 1983. "No meat lovers?" Dean rolls his eyes and digs his fingers into Sam's side, "Sure, we can get all of the vegetables your skinny butt desires." Ducking his head Sam beams and takes the hand Dean offers him.
The plan is simple. Dry erase markers are two for 99 cents at the gas station when Dean enters the next morning for an OJ and cereal run. The idea hit him while emptying his pockets at the register, if Sam can't see how amazing and special he is then Dean will just have to show him. The plan goes into action later that day while Sam is passed out on the couch. Ater Sam naps he will roll out of bed, rub at his eyes and make a beeline to the bathroom. When he gets there the words "you are worth it" will stand out on the dirty motel mirror. It's not enough, not by a long shot, but it's a start.
Later, when Sam steps out of the bathroom he has a smile and a blush on his face, and when Dean murmurs "okay?" to him later that night when they are tucked into bed, Sam answers with a content "okay" of his own. Maybe their life isn't perfect, and maybe it's not fair for Dean to have to raise his baby brother by himself at 18, but, in the end, this is all Dean needs to be happy. Their father needs to kill monsters to feel contentment. All Dean needs is a safe and happy baby brother, and Dean will do anything to keep their little family together.