It is upsetting when they misinterpret a smile. I was smiling once when I went in after an enjoyable time with a friend, and the woman thought I was smiling about seeing her. After letting her know she was wrong, I have made sure to be in an externally neutral frame before an appointment ever since. If I am uncertain, I remind myself of that incident and any hint of smiling is gone. I don't think it is useful for them to think clients are all happy to be at a therapy appointment.