Nightmares
Sam rolled over in his bed and blinked slowly at his brother, who had just fallen back to sleep after a nightmare.
Sam's mind slipped back to earlier times. When he'd first been thrust into the world of monsters. Times when Dean left on jobs with their father. Times when Sam would do research on the things they were hunting. Times when he would wake in the middle of the night from a nightmare, crying out for his older brother.
Dean would tell him that it was just a dream. It wasn't real. He'd tell Sam that he was there, he'd protect him from anything that came after him. Then came the time that Sam woke to the sound of his brother starting awake from a nightmare. Sam, of course, questioned what had occurred in it. What Dean had seen, what he'd experienced. And, as Dean always did, he brushed it off and told his younger brother to go back to sleep.
Sam had lain in bed for a while, trying to figure out, after all those nightmares Dean scared away, after all the monsters Dean fought, what could he have nightmares about? What constituted as a nightmare in Dean's book? What scared his older brother? Because if it scared Dean, then it must have been scarier than Sam could imagine.
As Sam rolled back over in the old motel bed with a creak and his mind settled back on current events, he tried to figure out what was worse; knowing that there was something out there that his invincible brother was afraid of, or knowing exactly what it was.