At 18 Sam Winchester left his father and brother to go to college but that was not the only reason. He left because something about his big brother, the one he'd always looked up to, was beginning to frighten him.

As a child the way Dean would talk sometimes about monsters, like they were real, had scared Sam. He would often say to Sam that a person walking across the street, or in a store, or at a diner, was a monster, a real one, like the boogeyman under the bed. When Sam told his father about what Dean said sometimes, John Winchester would tell the boy that monsters weren't real, of course. That his brother was just sick, confused, thats why Dean needed to take pills, and why sometimes Dean went away for weeks, or months, at a time to a special hospital.

As Dean grew up he talked less and less about monsters and by the time he was a teen, Sam no longer thought of his brother as being sick. Until Sam had walked in on Dean in the bathroom one day and saw his brother washing blood off his hands. At first Sam had been worried Dean had cut himself, but he quickly realized the blood wasn't Dean's. When Sam asked where it had come from, Dean merely smiled in a way that sent chills down the boy's spine.

Sam never told their father and not long after that, Sam's full ride to college came through and he left home. One day Dean shows up at Sam's college and Sam truly begins to understand how sick his brother really is.