It could also be that one of his Pokémon got seriously injured. Ash has a deep bond with his Pokémon, and seeing them in pain would surely make him not feel good. For example, if Pikachu got hurt while protecting him from a dangerous Pokémon attack, Ash would be very worried and sad.
One reason could be that he lost a really tough Pokémon battle. Losing can be a big blow to his confidence and make him feel down. Maybe he was facing a powerful opponent he had been preparing for a long time, and the defeat made him question his skills as a Pokémon trainer.
One reason could be that some writers want to create shock value. By showing Ash Ketchum being abused, they think they can get more attention from readers. Another reason might be that they are exploring different aspects of human nature in a very extreme way, even if it doesn't fit the original character's concept.
A possible theme is the betrayal by a mentor figure. Suppose an older, more experienced trainer that Ash looked up to gives bad advice on purpose, leading to his defeat. This would make Ash feel both betrayed and sad, crying over the loss of trust and the failure in his Pokémon journey.
One reason could be to pursue a new and more challenging adventure outside of the usual Pokémon journey. Maybe he feels he has achieved all he can in his current region and wants to explore uncharted territories where Pokémon training is different or more extreme.
One possible storyline could be that Ash has to go undercover in a dress for a special mission. Maybe he's trying to infiltrate an all - female Pokémon training group where men are not allowed. So, he dons a dress to blend in and learn their secrets.
His Pokémon might try to cheer him up. Pikachu, for instance, could use its cute antics to make Ash smile. It might nuzzle against him or do some of its signature electric tricks in a playful way. They have a very close bond, so the Pokémon would sense his sadness and do their best to make him feel better.
His rival, Gary, could be a likely candidate. Gary has always been competitive with Ash, and in this fanfiction, his jealousy could reach a boiling point, causing him to betray Ash for his own gain, like trying to steal his Pokémon or get credit for his achievements.
In this fanfiction, Ash's father might be a trainer who lost his memory. He has been traveling around the Pokémon world without realizing he has a son. One day, Ash might cross paths with him during his own journey, and through a series of events, his father's memory could be restored.
I'm not entirely sure as I haven't read a specific 'ash ketchum diaper fanfiction' myself. But it could potentially be a fan - created story that involves Ash Ketchum in some sort of situation related to diapers, perhaps in a humorous or out - of - the - ordinary way that fans like to imagine within the context of the Pokémon universe.
I'm not entirely sure as 'ash ketchum slave fanfiction' isn't a mainstream or positive concept. It might be some very unorthodox and inappropriate fan - created story that goes against the normal values associated with the Ash Ketchum character. It's not something that should be promoted.
One reason could be for the sake of adventure. If he's immortal, he can go on countless adventures without the worry of dying. It allows the story to be more continuous and expansive.