Rating: PG

Disclaimer: I don't own Supernatural

Summary: Castiel and Sam have a conversation about why Sam is Lucifer's Vessel

Author's Notes: This is somewhere around the events of "99 Problems" 5x17. Sam's desperate and Castiel's a bit lost.

Reason

"You know, they never said why I'm Lucifer's vessel and Dean is Michael's." Sam's voice is gutted and Castiel is sure he is shaking, slumped down against the side of their latest cheap motel. "Even though we have the same blood. And it's not the demon blood, not like I thought once. That's not the difference, not enough of one."

Castiel is cautious with his words, feeling something important depends on his response. Maybe the fate of the world,

Maybe keeping Sam breathing.

"Why Sam?"

The younger Winchester's lips twist up, eyes still straight ahead of him, dull like he's only half there.

"Dean wants to save the world, save humanity. Even if he has to say yes to Michael, he'll do it in the end if it means the world is spared." Glazed eyes drift downward. "I won't."

A moment passes in silence and when Castiel thinks Sam has drifted off somewhere he can't follow, the angel reaches out a gentle hand on his arm to pull him back. Sam takes a deep breath at the contact, like he hadn't been breathing in the interim.

"I wanna…save the world Castiel. I want to protect the happiness I know is out there even if I can't see it. I really do." His voice is pleading with the angel to believe him. He does.

"But…if it comes to the end…and I have to choose…" Sam bites his lip harshly. The lines of worry on Castiel's forehead deepen. "I'll pick Dean, Cas. I'll watch the world burn, drown it in ashes myself." The arm under Castiel's hand has gone from trembling to jolting as Sam sobs.

"I'll make him hate me, make him want me dead and destroy everything. I'll become our worst fear if it saves him." He has his head in his hands, tears dripping between his fingers. "I'm Lucifer's vessel…because I'll destroy the world, sacrifice every last person just so the one life I care about the most will be safe."

Sam curls down, forehead touching the floor.

"I'll be the monster they always said I was…."

The angel leaned forward, wrapping an arm across Sam's hunched back as he shook. Attempting to comfort him the way Dean no longer did.

For the first time in a long time, Castiel prayed.