It's frowned upon to be in a bad mood.
John is having a hard time at work and is in a bad mood because of it.
John wants to stop by his parents house for dinner.
John tries to suck it and be a little bit cheerful at dinner.
John stays in a sullen mood at dinner and brings down the whole room.
John has a good time and his mood is impoved.
His parents think something is wrong or everyone has a bad time.