The animal could be unhappy because it's stuck in a bad situation or facing some sort of danger. Sometimes, it's just a story device to make us feel for the character.
Maybe it lost its friend or didn't get the food it wanted.
Maybe she had a fight with her friends or failed a test.
The cartoon girl might be unhappy because she's being bullied at school or her pet is sick. Sometimes, it's a combination of small things that add up and make her feel down.
The cartoon cat could be unhappy because it lost a competition or had a fight with a friend. Sometimes, even small things can make a cartoon character feel down.
It might be that the dog in the cartoon is unhappy because it got scolded or had a bad experience during a walk. Or perhaps it saw something scary that made it feel that way.
Maybe he had a bad day at work or a fight with a friend. That could make him unhappy.
Maybe it didn't get its favorite treat or was left alone for too long.
Maybe it's because of the physical discomfort and stress during pregnancy. Or it could be due to some external factors affecting her mood.
Perhaps it's to make people think differently about the concept of a happy family. It might be suggesting that true happiness involves more than just the outward appearance and that there can be hidden emotions and challenges.
It could be part of a funny or unexpected plot twist in the cartoon.