If he wasn't expecting to be asked to dance, then he clearly doesn't know enough about parties.

Thanatos feels like he's fussing (which... is not an emotion geists ever really display, Ryoji just projects a lot), but Ryoji can't tell if it's because there's something wrong or if he would rather Ryoji were out stopping rituals or something. Which Ryoji would do if he knew about anything happening, but he doesn't, so there. "Why not?" he asks the masked man, smile evident underneath his own, "Come on, it'll be fun!"