A Gravity Thing.
It was never about lust. They just fit together perfectly. They just gravitated towards each other naturally.
There was never an awkward confession. They both just knew. They fell back onto the bed after a long hunt and shared lazy kisses as their father slept unaware in the next room.
No one ever knew. They kept this secret more guarded than the truth about hunting. They just seemed close to the other children at school. The teachers just dismissed their dependency as a reaction to their moving around so much.
Dean had always known. He'd known when he'd carried Sam from the burning house that night. He'd known when he made Sam dinner each night and tucked him into bed. He'd known when he'd lost his concentration during a fight because Sammy had been hurt. He'd known whenever Sam smiled, or laughed, or pulled a bitch-face. He'd known when his little brother slept peacefully, something he himself hardly ever did.
Sam had always known. He'd known when Dean would first yell at him, and then apologize and hug him. He'd known when his brother sacrificed going on a date to stay home and take care of his brother. He'd known when Dean made up stories for him at night, so that he could sleep without any nightmares. He'd known when it was always his brother to come running and calm him down when he did have nightmares. He'd known when his big brother would clean up each scrape, cut, and bruise Sam had with as much delicacy as he could.
They grew up together, relied on each other for everything, thought the world of the other. And just because everyone else couldn't see it, it doesn'tt mean it wasn't there. It had always been there. It would always be there. They both knew.
It was never about lust. It was simply a gravity thing.