The 'glad monster sad monster story' is likely about two different monsters, one that is happy (glad) and one that is sad. Maybe it shows their different daily experiences or how they interact with the world around them.
Perhaps Constance is a misunderstood monster in the house. The sad story might be that no one ever tried to understand it, and it was always feared and shunned. This led to a cycle of loneliness and sadness for the monster, and the house became a place full of melancholy.
Once in Monster School, there was a little ghost named Casper. He was always left out because he was too friendly and not spooky enough. The other monsters like the vampires and werewolves would not include him in their games. He felt so sad and lonely, always wandering around the school corridors by himself, longing for a friend who could accept him as he was.
Well, it could be related to the difficulties the Monster High characters experience in their relationships. For example, a love story that goes wrong due to some external factors such as family feuds between different monster clans. This would make for a really sad story as the characters would be torn between their love and loyalty to their families.
If a monster discovers that they don't fit in anywhere in the school. Let's say a hybrid monster, part - unicorn and part - troll. The unicorns don't accept him because of his troll side, and the trolls don't like him because of his unicorn features. He feels like he has no place to belong in the whole school, and this sense of isolation is extremely sad.
The concept of monster genetics in the Monster Tale story is really fascinating. It can determine not only physical characteristics but also the monster's behavior and intelligence. For instance, a monster with a genetic predisposition for high intelligence might be able to solve complex problems or communicate better. This genetic aspect could also be used to explain the evolution of different monster species over time in the story. It could be that mutations in their genes led to new abilities or appearances, which in turn affected their survival and success in the Monster Tale world.
The sad story could be about how the Constance Monster House was once a happy place. There were parties and laughter, but then an evil sorcerer put a curse on the house and Constance. After that, everything turned gloomy. The flowers in the garden died, and the rooms became cold and dark. Constance tried to break the curse but failed every time, and it was left in a state of eternal sadness.