Legit question. He's been through this with Ruby before, and no one listened to him. He wasn't convinced, or he wasn't surprised when it turned out she was playing Sam like a fiddle all along. How is he supposed to trust Meg now? To believe her? She's tried to squash out his and Sam's existence for a while now, and maybe this thing with Cas has been her way of weaseling in deeper. How is he supposed to know? Cas isn't exactly the greatest judge of character, though the fact that he was bonkers and reading her as safe, or making her actually uncomfortable with how sweet he was on her -- that's been something that Dean's thought about. Not that he wants to, but it makes it a little easier for him to accept them all living together.

He wants to play nice with Cas's demon friend here for his sake, but trying to believe that Meg's in love with him -- that's almost too unbelievable for him to take all at once.

"Let's hear the honest one." He reserves the right to still believe the other one.