One main theme could be the duality of emotions. We have the gladness represented by one monster and sadness by the other.
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.
Another theme could be the power of money. The money monster could represent how money can have a controlling and sometimes menacing influence over people. It might show how people are willing to take risks or change their behavior in the face of financial gain. Also, there could be a theme of consequence. If the characters interact with the money monster in a negative way, they might face negative consequences like losing their integrity or getting into financial trouble.
Well, in the Frankenstein monster story, a big theme is alienation. The monster is alienated from everyone around him. He doesn't fit in, neither with humans nor with the world he was brought into. Another important theme is moral responsibility. Frankenstein has a moral obligation towards his creation, but he fails to fulfill it. And there's also the theme of the power of creation, showing how a new form of life can have unforeseen consequences.
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.
One main theme is the battle between good and evil. The'monster' represents the evil force, while the'men' often symbolize the forces of good trying to combat it. Another theme could be justice, as the men may be seeking justice against the monster.
One of the main themes is the danger of overreaching human knowledge. Victor Frankenstein's attempt to create life goes against nature and brings disastrous consequences. Another theme is alienation. The monster is rejected by society and his creator, which makes him bitter and vengeful. Also, there's the theme of appearance vs. reality. People judge the monster by his hideous appearance without seeing his inner self.
One of the main themes is dealing with grief. The boy in the story is facing the illness of his mother, and through the monster's visits, he learns to come to terms with the impending loss. Another theme is the power of stories themselves. The stories told by the monster have a profound impact on the boy's understanding of his situation and the world around him.
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.
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.