Dean knows that he isn't the brightest, but he never cared much for school anyway. In his world, school never meant anything other than a distraction. He knows how to sniff out a monster, he knows how to find food in the forest and he knows how to kill better than he knows his times tables. If someone asked him who the sixth president of the United States was, he wouldn't be able to give an answer. If someone asked him how long it takes to load a Baretta M9, he could tell you to the exact number of seconds.
Dean knows that he isn't the brightest. Maybe in another life he could have gone to college, done something to make his mother proud, because he's always liked learning new things and he thinks he could have enjoyed the pattern of a 9 to 5. But his life has always been blood and bone and blades.
John Winchester was a flawed man and a worse father and it isn't until he's long dead that Dean starts to understand that - not the brightest - but even then it doesn't make him love his father any less. He was cruel and he was unforgiving but he always did what he could to keep them safe, even when that was no more than giving them a gun and teaching them how to use it. John was Dean's teacher. When the people at his ever-changing high schools were tearing their hair out because Dean couldn't recite his numbers in German, John was manipulating his fingers around a trigger, and telling how to sight down the barrel.
So Dean isn't academic, isn't a straight A student but he's spent more of his life missing school than attending. He can only do his best.
On long car journeys he'll curl around Sam on the back seat and read them stories until they fall asleep tangled up in each other. When Sam grows old enough to resent the contact, Dean reads to himself in silence, and pretends he doesn't notice how cold the car feels without his brother's warmth pressed to his side.
Sam is academic. As a young child, he shared his successes with Dean, rushing to him in school corridors, thrusting the red 'A' into his face and relishing the wide smile that bursts across his brother's face. No one in Sam's whole life is ever more proud of him than Dean. As Sam grows older, he relies less on Dean's smiles and more on his teachers' praise. It burns a sour hole in Dean's stomach when he realises that Sam no longer looks to him for encouragement.
But Sam is not like him, and Dean understands that - is grateful with every inch of his worthless soul for that - so he never once tells him how badly it hurts. If Sam wants more of a life than hunting can offer, then Dean must smile and help him get where he wants to go. It's Dean who offers to drive Sam to Stanford when he decides to go, when their father is seeking comfort in the bottom of a whiskey glass, and so it's Dean's heart that shatters when Sam turns up his nose and states that he wants nothing else from either of them.
(Sam caves moments later when he sees Dean's face crumple. He apologises for it and allows himself to be given a lift to the nearest train station.)
When Dean returns, it's to find his father passed out in a drunken sleep. He does his best to tuck him into the motel bed and try to stop him from choking on his own tongue but it's nothing more than a distraction. He's not had a room to himself since he was four years old and he might not be afraid of monsters but he's damn sure afraid of being alone. But if John wakes up to find him lingering in his room, he's in for another argument so he lets himself out quietly and returns to the twin room they'd gotten for him and Sammy.
Dean doesn't sleep that night.
Or the following one.
By the third day he's so tired that he passes out, only to start awake again half an hour later from dreams that he can't bear to recall.
They don't hear from Sam for two months and then they do and somehow, it's worse. Dean will always wish Sammy all the happiness that this shitty world has to offer but hearing how happy he is at Stanford and how much nicer it is there, away from his family, is almost enough to break him. Sam's been loving it while Dean hasn't had three hours of uninterrupted sleep since he left. But he doesn't say a word.
That night, Dean makes a list of all the things he knows.
He knows that he isn't the brightest. It's not a surprise when the list covers barely half the page. He knows how to load and fire all manner of weapons. He knows the habits and mortalities of creatures most people don't think about outside of nightmares. He knows the impala down to the last nick. He knows that whatever he might say, John always wanted the best for Sam. He knows that there is nothing more important on this whole goddamn Earth than his little brother.
Sam stops writing after about a year, but Dean never does. He can't write all that often and he's careful as to what he says in the letters - because he never knows who might read them - but he does his level best to send at least one letter a month. He'd made the mistake early on of sending Sam some money - he'd gotten a good haul from pool hustling - but the furious reply had stopped him from giving him anything else. (Though on Sam's birthday, he sends him all the money he has because he has no idea what he might actually want as a present. The fact that he no longer knows his own brother is enough to send him into a drinking spiral for over a week.)
(Dean hears nothing on his own birthday but that doesn't matter. He'd only even known it was his birthday because he'd happened to see the date on someone's paper. John doesn't even notice.)
When their father goes missing, Dean hesitates for days on whether or not he should call Sam - he'd want to know about this, surely - but when he finally builds up the courage to do it, the call won't go through. He shouldn't be surprised that Sam ditched his phone, but he can't help the ache that fills him when he realises that Sam hadn't bothered to give him a new number.
The whole time Sam had been gone, Dean had been adding things to his list. He now knows that he can last twenty minutes in the snow without a jacket before he's shaking too badly to aim. And that Winston owed Julia more than he gave her. And if he smirks in just the right way, he can get a woman into his bed three drinks earlier.
When Sam comes back, Dean wonders if he still needs the list. What does it matter, what he knows? He has Sammy. But he keeps in tucked into his jacket against his heart anyway because he finds that reading the words at 5 am make him feel a little less like a failure, and he might hate himself for feeling so weak but no one knows or cares enough to judge him for it.
Six months later, Sam finds the tattered scrap of motel paper when he's cleaning blood off one of Dean's shirts. Dean himself is in the other room, trying to sleep off a brutal concussion, so Sam can read the list in peace. By then, the list has grown to include 'I know that Sammy snores when he's been drinking' and 'I know that Bobby deserves a better son than me but I know that I couldn't do this without him.'
In tiny lettering on the back, Dean has also written 'I know that Sam doesn't want to be here and that he's going to leave again as soon as we find dad. I know that I can't blame him for it. I wish that I knew how to let him go.'
Sam tucks the note back into the jacket and pretends that he isn't crying over a stupid scrap of paper. Sam knows that Dean might not think it of himself, but he has the brightest soul Sam's ever seen and that nothing he ever does is going to make him worthy of his older brother's loyalty.
It's not going to stop him from trying.
The Winston and Julia thing is a nod to 1984 by George Orwell, to the people that didn't get it. The Dean in my headcanon reads a lot.
In case you couldn't tell, I'm in an awful mood.