Sam isn't scared of thunderstorms, exactly--an angry spirit trying to throttle you while it flings your brother across the room and drops a chandelier on your dad kind of puts a different perspective on the impersonal forces of nature--but when one comes close enough that each peal of thunder rattles the air in his lungs, and then just sits directly overhead for what seems like the entire night....
Sam also isn't really young enough to be crawling into his brother's bed anymore, but right now he just needs someone to hold onto so he doesn't get shaken to pieces by the weight of sound.
"Dean," he says during a lull, clutching at the edge of his brother's sheets. "Dean, please be awake." Because if Dean's somehow sleeping through this, Sam's a little afraid he might go mad.
But, "Yeah, okay," Dean answers, and rolls a little closer to the wall so Sam can climb into bed with him.
"'m not scared," Sam tells him, once they're settled--or as settled as they can be when the bed frame keeps juddering from the sheer volume of the thunder. "Just can't...." he doesn't know how to finish the sentence. Think? Sleep? Stop expecting the world to fall to pieces? "Just can't," he concludes, pressing his face into the safety of Dean's shoulder.
"Yeah, okay," Dean says again, voice fond, and rolls a little closer to Sam so he can cup the back of Sam's neck, thumb rubbing up and down in Sam's hair. After a minute or two he starts scratching his fingers through Sam's hair properly, and Sam can't help but go boneless.
Dean keeps it up until the storm finally begins to move away, by which point Sam is all but purring, so at ease that he doesn't even notice when he begins sliding back into sleep, almost missing Dean's murmured, "Okay?" when his fingers finally go still again.
"'Kay," Sam exhales, and when he opens his eyes the sun's up, the world is clean and fresh, and Dean's scrambling eggs in the kitchenette.