Michael: When I gave up Dean, you didn't think to question it, to ask why? Dean was... resisting me. He was too attached to you, to all of you. He wouldn't stop squirming... to get out, to get back. So I left... but not without leaving the door open... just a crack.

Castiel: Why wait?

Michael: To break him, to crush and disappoint him so completely that, this time, he'll be nice and quiet for a change... buried. And he is. He's gone. And now... I have a whole army out there, waiting, ready for my command, ready...for this. (snaps his fingers)