She got a lot of attention for being sad and withdrawn, perhaps more attention than she gets when she is a happy participant in family activities. If she is given attention for sulking, she is likely to continue to sulk. After all, sulking works for her. Sulking can give children a feeling of power. Everyone wants to figure out what is wrong and wants to fix it.
People often bend over backward trying to jolly a child out of a bad mood.
Everyone, even children, experiences occasional bad moods and periods of sadness. Knowing the cause of moods can help adults understand and adapt. The child in the fast food restaurant was sulking because she had not gotten a window seat in the van. This is not a major issue and the family could easily have ignored her. If this pattern of response continues, sulking can become an unpleasant habit for this youngster.