It could be that Meowth discovers some dark secrets about his origin. In fanfiction, there could be a plot where he finds out that he was part of an experiment or something similar. This realization that his life has been manipulated in some way could be very depressing for him. Or perhaps he witnesses the destruction of a place he holds dear, like a small town where he used to get food easily or a hiding place he thought was safe. This loss would surely make him sad.
One reason could be unrequited love. In the Pokemon world, Meowth has a soft spot for certain Pokemon, like the Skitty. If in a fanfiction, Skitty rejected Meowth's advances, it would make him sad. Another could be his longing for acceptance. Meowth is a part of Team Rocket, but often they don't fully appreciate his skills. He might feel sad and underappreciated in a fanfiction scenario. Also, memories of his past life on the streets, before joining Team Rocket, might haunt him and bring about sadness.
Maybe in the fanfiction, Meowth is separated from his friends. He has formed bonds with Jessie and James over time. If for some reason, like being caught by a powerful trainer or getting lost in a dangerous place, he is away from them, it would make him sad. Additionally, if he fails at a task that he really wanted to succeed in, for example, stealing a rare Pokemon for Team Rocket but getting outsmarted instead, that could lead to sadness. He is a proud Pokemon, and failure might hit him hard.
A writer can use Meowth's body language. For example, they can describe his drooping ears, his slow and slouched walk, and his tail hanging low. This physical appearance immediately gives the readers an idea of his sadness. Also, the use of simple dialogue like 'Meowth meow (I'm so sad)' can be effective.
However, over time, he could develop resilience. The experience of being abandoned might force him to become stronger, both physically and emotionally. He may learn to trust again, but it would be a slow process as he has to overcome the initial emotional trauma of abandonment.
Well, one possibility could be that Meowth betrays Team Rocket. Maybe he gets tired of their constant failures and decides to side with Ash and his friends. He might see that they have a more positive and successful approach to their Pokemon journey.
Another reason could be unrequited love. As I mentioned before, if Meowth has feelings for another but is rejected, that's very painful. It can lead to a long - lasting sadness.
It could be Giovanni. If Giovanni gives an order that Meowth finds completely unacceptable, like harming innocent Pokemon for no good reason. Meowth, having a soft heart deep down, might not be able to go along with it and thus betray Team Rocket.
One possibility could be that Meowth evolves after a deep emotional experience. Maybe it befriends a Pokémon that is in great danger and through the process of saving it, Meowth unlocks a new level of power within itself and evolves. This could lead to a whole new form with different abilities and a more noble appearance.
Meowth joining Ash in a fanfiction could also lead to some interesting character development for both. Ash might learn more about the Pokémon world from Meowth's perspective, especially the seedy side that Meowth has been a part of with Team Rocket. And Meowth could learn about friendship and loyalty in a new way, different from what he's experienced with Jessie and James.
Well, perhaps there could be a fanfiction where a powerful and mysterious Pokemon trainer captures both Meowth and Pikachu. The trainer forces them to battle each other for his amusement. But instead of fighting, they team up to escape. They use their unique skills, like Meowth's ability to speak human language to communicate with other Pokemon that can help them, and Pikachu's electric attacks to break through barriers. During their escape, they learn more about the trainer's dark secrets and also become closer friends.
In a fanfic, Meowth's evolution might be a result of a magical spell gone wrong. Instead of evolving into a normal form, it becomes a hybrid of different Pokemon types. This new form gives it strange and wonderful abilities, like being able to turn invisible or create illusions. But it also has to deal with the confusion of its new identity and how other Pokemon and humans react to it.
If Meowth joins Ash in fanfiction, it could lead to some really interesting character dynamics. Meowth has been part of Team Rocket for a long time, so there would be a lot of room for him to change and grow. He could teach Ash some of the sneaky tricks he learned from Team Rocket, but in a more positive way. Maybe they'd form a new team and compete in different Pokémon competitions.