Illness or injury can also be a factor. A sick or injured Meowth might not be able to do the things it normally does. It could be left behind by its group or not be able to fend for itself, which would be a really sad situation.
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.
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.
Meowth in Team Rocket has a sad story mainly because of his unrequited love for a female Meowth. He learned to speak human language in the hope of impressing her, but she still rejected him. This made him feel very sad and lonely, yet he continues his journey with Team Rocket.
Financial difficulties might also play a part. If they were in debt or lost all their savings due to some unforeseen circumstances, it could have caused a great deal of stress and sadness. Their relationships might have suffered as a result, and this could be the sad story of Jamich.
One possible reason could be the loss of a loved one, like in the case where the cat's owner died. Another might be separation from friends or family, as the cat that lost its mouse friend. Or neglect, such as when the family focused on the new baby instead of the cat.
One reason could be neglect. If the owner doesn't take proper care, like not feeding it enough or not providing shelter, the horse would be sad.
One reason could be the difference in their destinies. Silver is often tied to the future of his own world, while Blaze has her own duties in her dimension. This can lead to a sense of separation and sadness. For example, if Silver has to go back to his time to fix things and Blaze can't leave her world, it's a sad situation.
One possible reason is the pressure of always having to be perfect. As a virtual idol, Miku is expected to look and perform flawlessly all the time. Any glitch or mistake could be seen as a failure, which might make her sad.
One sad Meowth story could be that Meowth was in love with a female Meowth. But she rejected him because he could speak human language which made him different. He was left all alone, feeling heartbroken and out of place in his own kind's community.
One possible reason could be jealousy. If either A or J was a jealous person, it might have caused problems in their relationship.
One reason could be bullying at school. Other kids might pick on him because he's different in some way, like being too shy or having an unusual hobby. This can make him feel really sad and isolated.