Well, it's mainly to avoid any legal issues and to be transparent about the source of inspiration and any potential copyright concerns.
Maybe they just want to have fun and be creative without a specific reason. Sometimes you just go with the flow and write what comes to mind.
People might write this type of fanfic to challenge the norms of the original story. The Twilight series has a set relationship pattern, and by pairing Bella with James, they can break free from that. It gives them the freedom to re - imagine the world, the characters' personalities, and the overall story arc in a very different way. It also allows for a more villain - centric view in some cases, as James was originally a villain, and seeing him in a relationship with Bella can be an interesting exploration.
It could be a form of self - expression. Fans who identify with the LGBTQ+ community might see Dumbledore as a character they can project their own experiences and feelings onto. By writing gay fanfic about him, they are able to create stories that resonate with their own identities.
Some people write 'bitter Naruto fanfic' to explore the darker sides of the characters. In the original Naruto series, while there are hardships, it still has a positive overall tone. By writing bitter fanfics, they can show how the characters would react in more tragic or difficult situations.
People write gender bender fanfic for various reasons. One is for the sheer creativity of it. It's exciting to see how a story can be transformed when genders are swapped. Another reason could be to explore different character traits. Maybe they think a male character would have interesting female - like qualities if made female, or vice versa. It can also be a way to add diversity to the existing fanfiction pool and attract different readers who are interested in seeing these unique takes on the original characters.
People write 'izuku is a girl' fanfic because it offers a fresh perspective on the original story. It allows them to explore different character dynamics and plot possibilities. By changing Izuku's gender, the interactions with other characters, like Bakugo or All Might, can be re - imagined in new and interesting ways.
People write 'clove and katniss fanfic' because they want to explore different relationships within the Hunger Games universe. The relationship between Clove and Katniss in the original story is complex and competitive, and fanfic writers like to play with that dynamic.
Some people might write 'love you' fanfic because they enjoy exploring romantic relationships in the context of their favorite fictional universes. But if you hate it, it could be that you find it too cheesy or unrealistic. Everyone has different tastes.
People may write it as a form of creative rebellion. The Pokemon franchise is so popular and has a very set image. By creating 'anti pokemon fanfic', they can break the mold and create something that stands out. It could also be a way for them to express their own views on things like animal rights (comparing Pokemon to animals in a sense) if they think the way Pokemon are treated in the franchise is not ideal.
There are several reasons. One is to create humor. The contrast between the serious and magical world of Harry Potter and the rather silly concept of diapers can make for some very funny stories. Another reason could be to explore themes of regression or new beginnings. Diapers can symbolize a fresh start or a return to a more innocent state. And for some fans, it's just a way to stand out in the large and diverse fanfic community by coming up with a really unusual concept that still ties back to the much - loved Harry Potter franchise.