The Mormon girl I dated in high school also tried to always appear in a good mood, put together, etc… but she really struggled, and had a lot of emotional outbursts while we were together. She even hinted at suicidal thoughts once or twice. Years later I realized she was probably dealing with some serious mental health issues. But the reason for the happy face is simple: they’re taught that if you’re unhappy that’s because of a lack of faith in the holy spirit. If you are truly faithful then it will always fill you up with happiness and positive energy. So letting other people see that you’re sad or upset about something will make them question your faith and dedication to the church. Even admitting it to themselves makes them ashamed, like they’ve committed a terrible sin. It’s really sick.