I am sorry you had a rough day at the dentist. In terms of your affect matching, I think it's something you can learn to be mindful of. Like if you are feeling really sad and crying, maybe take a moment to recognize what you're feeling and just admit to the person with you that you are sad/scared/etc and that you're okay, and it's just an emotional reaction. You can catch yourself if you're laughing it off or trying to fake your way through it and kind of mindfully stop doing that, I think. Might not be easy, it's just what has worked for me if I do that.