Well, sometimes it's a test of character. The magic might turn a person into a girl to see how they would handle different situations and emotions. It could also be part of a grander plan by a powerful magic - user. They might want to use the transformed person to achieve a certain goal, like infiltrating a group that only accepts girls. And in some stories, it could be a way for the character to gain a new understanding of the world and those around them, so the magic makes this transformation happen.
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.
In many stories, it can be a result of meddling with forbidden magic. If a character ventures into an area of magic that they are not supposed to touch, like a magic well that has specific rules, they might be turned into a girl. Also, it could be a reward in a strange way. For instance, if a character has shown great kindness towards a female - centered magic entity, the entity might turn the character into a girl as a way to let them experience life from a different perspective, which could lead to personal growth and new relationships.
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 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.
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.
A strange scientific experiment gone wrong. Scientists might be messing with genetic or energy manipulation and something unexpected occurs. Like they were trying to create a new form of human evolution but instead turned a person into a baby girl because of a miscalculation in the experiment's variables.
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.
It could also be due to past experiences with men that were not positive. If she had bad relationships or experiences with men that made her feel unfulfilled or unhappy, she might be more open to exploring relationships with women. Also, some psychological research suggests that there may be some inborn factors that could play a role, but it's still a complex area of study.
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.
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 possible reason could be a magical curse. Maybe she offended a powerful sea witch who then turned her into a mermaid as punishment. Another reason could be a strange accident involving some sort of enchanted water. For example, if she drank from a forbidden pool in an old, magical forest that was somehow connected to the sea. Or it could be a result of a selfless act. Perhaps she saved a mermaid's life and in return, the mermaid magic transformed her into one of their kind to join their underwater world.
Well, in a story, the friend might be playing a prank with some new technology they don't fully understand. There are some fictional technologies that can manipulate the body's physical form. Also, the friend could be under the influence of a curse or a hex that causes them to involuntarily turn others into different genders. It could also be that the friend is exploring some sort of spiritual or esoteric practice and makes a mistake during a ritual that results in the transformation. For instance, if they are trying to communicate with a gender - changing spirit and things go awry.