In some stories, it might be the result of a magical object. For example, if the boy touches an ancient amulet that has the power to change one's gender. Also, it could be related to a spiritual journey. In certain mythologies, a boy on a spiritual quest might encounter a spirit or a deity who decides to transform him into a girl as part of his learning and growth process. This transformation would then lead to him experiencing life from a different gender perspective, which is often a central theme in such stories.
Magic could be one reason. Like in a fairy - tale world, a magic spell might be cast on the boy. Another reason could be a scientific experiment gone wrong. Maybe some sort of genetic - altering experiment that was supposed to do something else but instead turned the boy into a girl.
It could be due to a curse. For instance, in a mystical land, the boy might have offended an old witch or a powerful being, and as a punishment, he was turned into a cute girl. Or perhaps it was a self - transformation out of a strong desire. The boy might have been so envious of the girls' world that his own intense wish triggered some sort of magical or supernatural transformation.
A scientific experiment gone wrong. If he was in a science - fiction type of setting, perhaps some crazy scientist was testing a new device or formula that had unexpected side effects of gender transformation.
Some kind of scientific experiment error. For example, if there was research on hormonal or genetic manipulation and something went wrong during the process.
One reason could be revenge. Someone might want to take revenge on the boy or his family by changing his gender. Another reason could be a crazy experiment. Some mad scientist - like character in a story might think it's an interesting experiment to see how a boy would adapt to being a girl.
In some turned into a girl stories, it could be due to a curse. For example, if someone offended an old witch or a powerful spirit, they might be cursed to change into a girl. Also, there could be a strange natural phenomenon. Like in a place full of strange energy, a person could be transformed into a girl just by being there for a while.
Magic potions. In fairy - tales, magic potions often cause strange transformations. A boy might accidentally drink a potion meant for something else and turn into a girl.
Magic is a common reason. Maybe a mischievous fairy or a clumsy wizard cast a spell wrongly. Another reason could be a strange scientific experiment gone wrong, like some sort of radiation or chemical reaction that alters the body's form. Or it could be a curse from an ancient artifact. If someone disturbs an old tomb and takes an enchanted item, it might put a curse on them and turn them into a little girl.
One possible reason could be a magic spell. Maybe a wizard or a witch cast a spell on the boy for some reason, like revenge or as part of a test.
Magic gone wrong is a common reason. Just like a magician who miscalculates a spell and accidentally turns someone into a baby boy. Another reason could be a curse. For example, if someone angers an evil witch, she might put a curse on that person to turn them into a baby boy as a form of punishment.
One possible reason could be that the parents have some unfulfilled desires related to having a girl. Maybe they always dreamed of having a daughter and are trying to create that situation by changing their son's appearance or upbringing. But this is a wrong approach as it doesn't respect the child's identity.
Family problems can also play a part. For example, if there's abuse or neglect at home, it can make her lose faith in goodness and turn bad. She may act out in rebellion as a way to cope with the pain.