That earns an incredibly suspicious look from Castiel, because he's not even honest with the Winchesters a lot of the time. Sure, maybe if he were younger and had spent three years with someone it's possible he might be more open, but at the moment he finds it incredibly difficult to believe and he doesn't know if Crowley is serious or attempting to play him somehow.

So he just watches the other for a moment, waiting to see if he'll elaborate.