Dean was possessed by Michael and he was fighting to regain control.

"Dean is resisting me. He's too attached to you. To all of you. He wouldn’t stop squirming, to get out, to get back." Said Michael, his voice flat.

"I'm thinking if we take away that source of resistance... then there's not much left for him to fight for." He tilted his head to the side slightly.

"Shall we give it a try?" A smirk appeared on his face.

It was a last resort for Michael but he was willing to get it done.