Disclaimer: Supernatural belongs to Eric Kripke, and the lyrics to Led Zeppelin.
Stairs made Castiel sad. He could say why, but that might ruin the song. And it was one of Dean's favorites. Mostly, though, he didn't want to think about it. So, he avoided stairs.
It was easy at first, since he could just use his grace to get around, but the Winchesters were a very observant pair. Sam more so than Dean, so next time they had to scale an abandoned motel, Sam asked about it.
"Wait," Sam says before Cas had the chance to wait for them on the top floor. Dean had already scaled most of the steps, but stops on the last one so as not to be caught without back-up. He looks down at the angel and his brother.
"What?" Cas asks, trying to sound as if nothing was the matter, just as he'd seen the brothers do time after time. It takes one to know one, and Sam saw right through him.
"There's something wrong. It's the stairs. You only zap around when we have to go up 'em."
"I thought you were just lazy," Dean adds from his perch. He warily checks the hallway at the top before returning to the stairwell. "Nothin' up here, I'm comin' down."
Cas winces. Sam notices. Dean does, too.
"Tell us," Dean says.
"Stairs," Cas begins.
"Yes?"
"Do you know where they go?"
"The second floor."
"They go up. And what is up?"
Dean looks at him for a while, but Sam glances up the stairs himself. When he turns back to Cas, his face is knowing and sympathetic. Dean looks at him for an answer. Sam starts singing.
"…and she's buying a stairway to Heaven."