Based on Season 1, Episode 5 "Bloody Mary"
Prompt: "Look, you're my brother and I would die for you, but there are some things I need to keep to myself" - Sam
Sam can't tell Dean just what the secret is. It's more than just watching Jess burn; years more. He had only dreamt about her death for week before it happened, but his mind had been cheating on her since the minute they met.
She was pretty, confident, funny. She was everything Sam needed to take his mind off his past and his unconventional family. But just days into their relationship he knew it would never be enough.
He glanced across the front seat of the Impala – Dean's face was worried into concentration, and concentrated into worry. His eyes betrayed his feeling of protection over his brother, his uselessness at not being able to help him; his frustration that Sam couldn't trust him enough to tell him the secret.
A secret worth dying for.
Sam glanced out of the window and for a second thought he saw Jessica there, watching him accusingly. Had she known? When Dean had shown up at their house had her stomach plummeted in realisation as his had filled with butterflies?
He had never been able to fight it, not since he was twelve and finally knew about all the times that Dean had saved his life.
He had killed Jess years before he met her – killed her because she was always going to be second best. Second best to his big brother.