Protection.
Dean promised himself that Sam would have a better childhood than he had.
He couldn't change their roaming ways- they were hunters, he knew that- he probably knew it better than a little boy should have- probably took on more than a 9 year old was meant to. He couldn't change the fact that dad left for maybe days on end without contacting them, couldn't change the fact that whatever dingy motel they were staying in smelt always of stale spaghetti-o's, heated inside the room's coffeepot over the little base-warmer, designed to stop the coffee going cold, never intended to cook dinner for two growing boys. He couldn't change the fact that they dressed in salvation army cast-offs, thrift-store bargains, military surplus and even, already, John's old clothes, the family hierarchy cemented in the passage of the garments- John, Dean, Sam.
He couldn't change much, but he could change one thing.
When John came home, he could keep his mouth shut, he could make sure Sam's belly was full, which always put him into a deep, unwakeable sleep. When John pulled Dean in to talk to him about his day he could nod, all too happy to keep to the low tones John dictated.
When John pulled Dean onto his lap, held him close and rocked him as his father cried onto his son's shoulder from too much seen, too much felt, too often, without enough time even to breathe in between, Dean could wrap his arms around his father, giving him what little comfort he could- he could concentrate on his love for his father, his family, to the extent that his mind blanked out to all else until he focused it and imagined beaming it out into his father's head, anything to stop the crying.
The crying was sometimes enough, John would take a deep breath, give Dean an extra squeeze and put him down on the couch beside him, ruffle his hair and turn on the t.v, and that was it for the evening, or however long John decided they could stay in one place.
But the other times, Dean could protect Sammy, just like he always did, by staying quiet and not moving too much. When John's tears changed to the dry sobs, echoing in his chest and his hands held Dean a little too close, moved a little too lingeringly over his body, he could bite his lip and shut his eyes and pretend he was somewhere else. He shut his eyes and thought of Uncle Bobby's place, where him and Sammy would be handed one of Bobby's pistols and taken out to the junkyard to shoot at cans. Bobby would guide their hand, he taught them to aim, take a deep breath, hold it, then gently, gently squeeze the trigger. He'd painted a target on an old sack, stuffed full of straw, and showed them how if you didn't hold your breath your arm would move as you breathed, making your shots go down in a diagonal line. They shared the little bedroom upstairs and dad talked with Bobby downstairs all night and he was safe, and warm.
He thought of Bobby's place, and he took a deep breath and held it as John turned him over, still crying, he took a deep breath and held it, staying silent as his legs chafed against the pleather of the sofa. When he was done, John held him tight, far longer than Dean had ever imagined possible- whole hours had passed sometimes, and all the while John would be taking deep, shaky breaths, whispering his apologies, mingled with sobs, telling Dean what a good boy he was, how he didn't mean to hurt him- he just lost himself sometimes and Dean could almost believe it as his father whispered to him, all the things he never did when they were hunting, working, driving, being…normal.
Dean would fall asleep, eventually, sore and shaken though he might be, lulled by the rustling susseration of John's breath in his ear, the half-hissed-half-whispered apologies. When he woke up, John would be gone, Dean's clothes righted, and that was when he knew dad would be gone for a long time- in a way he was glad, it always took him a few days to feel right in himself after John had one of his nights- he didn't know how he'd react to his father in the cold light of day. On the other hand, he felt lost, sad, alone- he didn't know what to do with himself. When he had a problem, he took it to John. And this aching pit in the middle of his belly, filled with shame and spite and fear was the biggest problem he had, the one thing he wanted to talk to his father about and the one thing he never could.
The night after John left, he'd be extra careful of Sam- his little brother could never understand why their dad would come home for the one evening and then go, and Dean couldn't stand to tell him it was because of him- because he was making his dad feel…wrong, or something- he wasn't sure why, he just knew that when John touched him he had to go, like Dean was the disease and isolation was the cure. So when Sammy cried because he missed their dad, Dean would climb into bed with him in one of the big motel beds and wrap his arms around him tight, the way John did when he was crying, and he'd hold onto Sam all night, until his little brother stopped crying and snuggled up against him, his breathing deep and slow, lost in sleep.
Then Dean might let himself take advantage- he'd keep his own tears silent- he'd learnt at a young age that John wouldn't tolerate whining. When mom died, and Dean had been sat in the back seat of the impala, sobbing, some 2 weeks later, John had turned around and smacked him around the face. He'd stopped crying instantly and John had looked him in the eye and told him to stop that noise. Everyone, he said, lost their parents eventually- death was inevitable, he told him- crying over the inevitable was unmanly and disrespectful toward everyone who had to hear his noise. So Dean kept quiet, and he muffled his tears into Sam's hair as they slept close together.
Dean stayed good for John, so that Sam never had to know, never had to worry, about how much their dad still mourned Mary, still thought about the home they could have had, never thought about how the two of them might attend 6 schools in 3 weeks in as many different states. Sam didn't need to know that dad cried- it was unmanly, and Sam needed to know dad was strong, so Dean was strong, for dad and for Sammy. He folded himself up in the cocoon of knowing how much he helped the family, how he could be their rock, the paper clip that kept everything neat and tidy and together.
So when Sam cried, he knew how to make him feel better- John had taught him the way to stop tears, and he'd use it to help Sammy- it made John feel better, so it could help his brother, and as they stumbled through the steps Dean, too, felt a little of the weight of the secret lift.
He could make sure that Sam had a better childhood than he had, John would never hurt Sammy, never touch him. Dean would protect him.