One possible reason could be that Pikachu is separated from Ash. In many fanfictions, their bond is so strong that being apart would be extremely distressing for Pikachu, leading it to cry.
In some fanfictions, Pikachu might be bullied by other Pokémon. Since it is often depicted as a kind - hearted and friendly Pokémon, being bullied would be very upsetting. This could make Pikachu feel sad and start crying. It might be excluded by a group of Pokémon because it's different in some ways, like having a special power that others envy or fear.
Maybe Pikachu gets lost in a dangerous and unfamiliar place. It's a small and vulnerable Pokémon in such a situation. It could be scared and cry out of fear. For example, if it accidentally wanders into a cave full of menacing Pokémon, it would be terrified and start crying.
Using Pikachu's unique electric abilities in the description of its crying can be effective too. For instance, the tears could be described as having a slight electric charge, sparking a bit as they roll down its cheeks. This ties in with Pikachu's nature as an Electric - type Pokémon and makes the crying scene more unique and memorable.
Maybe Pikachu is feeling neglected by its trainer. If the trainer has been paying more attention to other Pokemon or is too busy with something else, Pikachu might decide to run away to get some attention.
One common reason could be jealousy among other Pokemon. Some Pokemon might be jealous of Pikachu's closeness to Ash and its popularity. So, they might plot against it and end up killing it. Another reason could be a misunderstanding. For instance, a wild Pokemon might misinterpret Pikachu's actions as a threat and attack it fatally.
Perhaps Pikachu is lured away by something more appealing. There could be a rare and powerful Pokémon or a special kind of food that distracts Pikachu from protecting Ash. Also, Pikachu might have received some false information. For example, it might have been tricked into thinking that Ash has a better chance of escaping the Spearow without it. In the heat of the moment, Pikachu makes a hasty decision based on this wrong information.
One popular type of Pokemon Ash and Pikachu fanfiction might be adventure - based. For example, Ash and Pikachu could embark on a journey to a new and mysterious region filled with undiscovered Pokemon species. They might face challenges like difficult terrains and powerful, previously - unseen Pokemon. In this fanfiction, Pikachu's electric powers could be enhanced in some unique way, and Ash would have to strategize differently to overcome the obstacles.
One possible plot could be that Pikachu is exposed to a rare form of electric energy in a thunderstorm that's charged with some mysterious power. This could start the evolution process. It might struggle at first, not understanding the changes its body is going through. As it evolves, it could gain new abilities like enhanced speed and a more powerful electric attack. And it might have to face new challenges in battles, as opponents are now more cautious of it.
Another possibility is a rival trainer who has a Pokemon that is weak against Raichu. This trainer might try to prevent Pikachu from evolving to keep their own Pokemon's advantage in battles. They could try to steal Pikachu's food or training resources to slow down its growth and evolution.
Another reason might be related to Pikachu's own growth. As Pikachu becomes more powerful and self - aware, it might feel that it needs to go on its own journey without Ash. It could be seeking new challenges and experiences that it believes Ash can't provide, like exploring regions that Ash has no intention of going to or training with Pokémon of a different caliber.
In this kind of fanfiction, Ash might find it really hard at first. He's used to being a human trainer, and suddenly becoming a Pikachu would be a shock. He'd have to learn how to control Pikachu's electric powers which he only used to command before.
It could be about their adventures together. Maybe they team up to explore a new region, facing various challenges and making new friends along the way.