"Because I don't trust anyone else." The smile drops off his face and the answer is quick. He doesn't even have to think about it. This is fact. This is his life. Crowley trusts no one fully, because that's how he dies. That he trusts Blake as much as he does is a sign of the end, as far as he's concerned. It's the Crowley apocalypse.

"Because shit happens, people change and they can leave and come back wrong at any time." He moves away, back to Blake. "You could leave and come back with a knife ready for my heart. I could leave and come back a bloodthirsty monster. We can't predict these things and we certainly can't predict the people out there."