One possible reason could be a curse. Maybe the character in the story offended some powerful being, and as a punishment, they were turned into a girl. Or perhaps it's a result of a strange natural phenomenon. In some fictional worlds, there are places where strange energy exists. If a person enters that area, it could cause a physical transformation, turning a male into a girl.
It could be a deal with a supernatural entity. For example, in a story, a man might be desperate for something, like wealth or power. He makes a deal with a devil or some other otherworldly creature. As part of the deal, he is turned into a girl. Also, in some mythologies, there are gods who have the power to change a person's form, and they might do so for reasons related to their own agendas or the person's actions.
In some stories, it could be due to a magic spell. Just like in fairy tales, a wicked witch or an inexperienced wizard might accidentally cast a spell that changes a person's gender. Another reason could be a scientific experiment. Scientists might be trying to do something revolutionary like gender transformation through some new technology, but things go wrong and a male turns into a girl.
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 be due to a scientific experiment gone wrong. For instance, if he was involved in some kind of genetic or body - altering experiment and there was an unexpected side - effect that caused the transformation.
In some stories, it could be due to a deal with a magical creature. Maybe a mermaid promised to give a boy the ability to breathe underwater if he agreed to a transformation into a girl for a short period. Or it could be part of a magical quest. A boy might need to turn into a girl to infiltrate an all - female society and learn their secrets in order to save his kingdom. Sometimes, it could just be the result of a magical place. A meadow filled with enchanted flowers that have the power to change one's gender when one steps into it.
One reason could be for a performance, like in a play or a movie. Just like in Shakespeare's plays where male actors used to play female roles and had to be dressed in girl clothes. Another reason might be for a bet or a dare among friends. For example, if someone lost a bet, they might have to wear girl clothes as a consequence.
It might be that the man made a deal with a witch and broke the terms, causing the transformation. Or it could be a symbol of his inner transformation and represented metaphorically as a physical change into a dog. There could also be a virus or a strange potion involved.
One possible reason could be as a form of punishment. For example, if a character has been misbehaving in a way that is considered inappropriate for their gender, magic might transform them to teach them a lesson. Another reason could be an accident. Maybe a young wizard was practicing a spell and it went wrong, resulting in the transformation. Or it could be a magical curse placed on someone because of a feud between families or kingdoms.
One reason could be for a performance like in a play or a movie. Just like what I said before, an actor might be required to dress as a girl to play a certain role. Another reason could be for a costume party. People often choose unique or cross - gender costumes to have fun at parties. Also, in some cultural or traditional events, there might be the need for someone to dress as a girl for symbolic purposes.
In a tf story, it could be due to a scientific experiment. Scientists might be trying to find a way to give humans the abilities of Pokémon. The girls could be test subjects, and something unexpected happened during the process. Also, it could be related to a curse. If they entered a forbidden area or angered a powerful Pokémon - like spirit, they could be cursed to turn into Pokémon. This would add an element of mystery and danger to the story.
It's possible that the witch is just being mischievous. She might not have any real malicious intent but just enjoys using her magic to create chaos. Turning someone into a girl is just one of her ways to stir things up in the world around her.
One reason could be the strong emotional bond. If a lesbian and a straight girl form a very close relationship, the straight girl may start to see love in a new way. For example, she might realize that the deep connection she has with the lesbian is different from her previous relationships with men.