Another reason might be a personal exploration of gender identity. Some boys may feel that they identify more with female characteristics and may choose to present themselves as girls, either temporarily or permanently. It could be a journey of self - discovery and acceptance.
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.
One possible reason could be cultural or traditional beliefs in some extreme cases. For example, in a very patriarchal society where having a girl is highly desired for certain family inheritances or religious practices, a family might try to force a boy to live as a girl. Another reason could be related to some psychological issues of the people forcing it. Maybe they have a distorted view of gender and think they can 'create' the gender they want.
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.
Sometimes, it could be due to a lack of communication in the relationship. If she can't talk to the boy about her feelings or problems, she might look for someone else to confide in. Also, peer pressure or the influence of her friends can play a role. If her friends encourage her to be with someone else or make her feel like she can do better, she may be more likely to cheat.
One reason could be that she has fallen out of love with the boy but is too afraid to break up. So she cheats instead. For example, if the relationship has become dull and she meets someone new who seems more exciting.
In some stories, a scientific experiment gone wrong could lead to a boy transforming into a girl. For example, if a scientist was experimenting with genetic alteration and something unexpected happened. Another possibility is a wish granted by a mischievous or well - meaning supernatural being. Maybe the boy made a wish without fully understanding the consequences and ended up being transformed into a girl.
One reason could be that they have different long - term goals. For example, one may want to travel the world while the other wants to settle down immediately. Another reason is that trust might be broken, like if one of them cheats. Also, growing apart is common. As people change over time, they may find that they no longer have the same connection as before.
In some cases, it could be due to a lack of communication. If the girl felt that the boy wasn't listening to her needs or she was unhappy in the relationship but couldn't express it, she might turn to someone else. Additionally, peer pressure or family influence could play a role. If her friends or family didn't approve of the boy, she might be influenced to end the relationship in a wrong way by getting involved with another person.
Magic could be one reason. Like in many fantasy stories, a magical spell or a enchanted object can cause the transformation. For example, a magic amulet that when touched by a boy, starts the process of turning him into a girl.