Crowley talked Castiel into riding in the car.
"Come on, kiddo. It'll give you a chance to relax."
"I do not need to relax."
"…Sure. It'll give you a chance to watch over the Winchesters."
Castiel tilted his head in confusion.
"Well, I know I'm going to ride in the car. You're leaving your boys in my hands, and I am the best tempter in existence. I could tempt the lord himself into eating an apple. Are you really going to leave them with me?"
Castiel glared, and slowly got in the car.
Crowley turned with a sly smile and winked at Dean. "See, it's all about knowing where to push."