Another possible reason could be a curse. For example, an angry witch might put a curse on a girl as revenge, and that curse could turn her into a boy. This is a common trope in many fantasy stories where the main character has to find a way to break the curse and return to their original form.
In some fictional stories, scientific experiments gone wrong might lead to such a change. Maybe a scientist was trying to develop a new gene - editing technique and something unexpected happened to the girl who was part of the experiment.
One reason could be magic. Like in fairy tales, a magical spell might be cast on the girl accidentally or on purpose by a wizard or a magical creature. Another reason could be a scientific experiment gone wrong. Maybe she was part of an experiment that aimed to test some new genetic or hormonal treatment and it had unexpected results. Also, in some fictional worlds, a deal with a supernatural being might lead to such a transformation, for example, making a deal with a devilish figure who twists the terms.
In some stories, it could be a curse. Maybe the boy's family angered a powerful being, and as a punishment, the boy is turned into a girl. This then sets up a story of trying to break the curse and restore his original form, which often involves a long and difficult journey filled with challenges.
In some fictional stories, a curse can also be the cause. Maybe an evil witch or wizard put a curse on the man because of some past conflict or just for their own malicious purposes.
It could be due to a curse. Maybe someone with malicious intent put a curse on Ash that turned him into a girl.
Well, it might be due to a powerful magical artifact. She could have accidentally touched or used an enchanted item that had the power to turn humans into animals, like a cursed amulet. Or, in a more spiritual sense, it could be a connection to a spirit world where cats are guardians and she has been chosen to take on that form for a special mission. There could also be a deal made with a supernatural entity that she has to fulfill, and the transformation is part of the deal.
Another reason might be related to some kind of spiritual or mythical connection. In certain mythologies, there are beings that can transform between human and animal forms. She could be related to such a being, and due to some trigger, like reaching a certain age or entering a specific place, she turns into a cat.
Well, in some stories, it could be a family curse that has been passed down through generations. Maybe her ancestors made a deal with dark forces long ago and as a result, she is doomed to turn into a werewolf. Also, it could be related to a place. For example, if she lives in an area that has a strong connection to werewolf lore, the energy of the land could somehow trigger the transformation. And of course, as mentioned before, a werewolf bite is a classic cause.
One possible reason could be revenge. Maybe the girl felt wronged by the boy or his family in some way and decided to kidnap him as payback.
Magic could be a reason. Like in a fairytale world, a magic spell or a magic object might cause the transformation. For example, a magical amulet that when activated changes the boy's gender.
It could also be part of a reincarnation story. Maybe in his past life he was a girl and due to some karmic imbalance or a sign from the universe, he is transformed back into a girl in this life to learn certain lessons or fulfill a certain destiny. This could involve a spiritual journey where he has to come to terms with his new identity and the responsibilities that come with it.