He knows his brother is smart. He can calculate the miles of each of their trips in a matter of seconds, he can hack into any computer system necessary to get their information, he can create a false life will all necessary details to make it believable. He can charm anyone with his wit while Dean uses his smile.
When Dean allows himself to think about it he knows his brother's a genesis. But he also knows what that means.
When they hunt it's all grime and grit. Running through the forest in the dark of night, getting bloody and bruised fighting invisible demons, saving people who can't properly thank them. Dean knows his brother was meant for more.
So when his brother turns 20 and corners him in the dirty bathroom, Dean panics. He always thought he was better at hiding his desire for him. But apparently not, Dean thinks as his baby brother grinds his hips against his, his hand roaming across his taunt chest. His body reacts to the motions before he had resist. Yet he still can't bring himself to lift his hands to return the caress.
"Please Dean," His Sammy moans, his lips finding Dean's neck.
Dean clamps his eyes close, for once being the smarter brother. If he accepts, if he returns the desire it's over-for both of them. Deans knows his Sammy's meant for so much more….more then his brother in a gritty bathroom, more then running after ghost and demons.
"No," It's the single hardest thing he's ever done because he knows he just pushed his brother away for good. There was no going back after this.
He's relieved and heartbroken in the same instant when Sam moves away, tears stilled in his eyes. He stays in that bathroom for a half hour, his fist curled in his mouth, biting back sobs.
They don't speak again until Sam and his father blow up days later.
It's the worst fight Dean's ever seen between them. He stays on the sidelines, knowing the words of anger and hate are half directed at him.
He shares his father's shock and fear when Sam throws a letter at John. His full scholarship to Stanford. Dean watches in shock and horror as his father throws the letter back, yelling curses and threats.
That night Sam's sitting on the front porch of the cabin they were renting out. Dean steps out into the cool night and sits beside his brother. He no longer feels the awkwardness of rejecting his brother, it's all to surreal to believe it's happening anymore.
"How long have you known?" He muttered, his eyes on the full moon.
Sam sighs, "Two weeks."
Dean freezes, "So then why did you-" he cuts himself off.
Sam finishes for him, "Why did I try something with you?"
Dean hears the bitterness in his voice, but can't bring himself to say anything.
"Because I wanted to see if I was loved in the same way I love you."
Dean's mind swirls with the meanings of that. Incest is wrong, pure undulated sin. Disgusting. Evil.
His mind his still trying to grasp the entirety of his brother's statement when Sammy lifts himself from the porch.
"Goodnight Dean."
It's only days later, watching his brother climb aboard a train, backpack filled with all his belongings, Dean fights against the urge to call to his brother. To stop him and confess his own feelings. But Sam steps aboard the train, and Dean knows he's done what's best for him.
He knew all along that his brother was too good for this life, far to innocent and bright to be wasted on hunting. He knew all along that if he ever wanted to prove to himself how much he loved his little brother it would be letting him go.