One possibility is that he's trying to prove a point. Maybe there's some discrimination or unfair treatment towards girls in the wizarding world that he wants to highlight. By dressing like a girl, he can put himself in their shoes, so to speak. Another reason could be that he's influenced by a new friendship or relationship. He might have a close female friend who has inspired him to try dressing like a girl, either as a form of bonding or because he admires her style.
It could also be part of an experiment. He might be curious about how it feels to wear different types of clothing. Or perhaps he's been influenced by some new magical culture he's learned about that has different views on clothing and gender. There could be a spell or a potion that makes him more in touch with his feminine side, and as a result, he starts wearing dresses.
Maybe Harry could be on the run from some dark wizards and he disguises himself as a goth girl to avoid detection. He would have to learn how to act and move like a goth girl too, which could lead to some comical situations.
One possible reason could be for fun. Maybe she just wants to see how cute her son would look in girl's clothes for a laugh or a unique photo. Another reason might be that she doesn't have a strong sense of gender - specific clothing. She might think that clothes are just clothes and doesn't see a big problem in dressing her son in girl - like outfits.
One possible reason could be that the mom has always wanted a girl. Maybe she had dreams of dressing up a daughter in cute frilly dresses and bows, and when she had a boy, she couldn't resist the urge to fulfill some of those desires in a way. Another reason might be that she wants to break gender norms. She could be trying to teach her son that clothes don't have a gender and he can be comfortable expressing himself in any way he likes.
In a story, a boy might dress like a girl because of a bet or a challenge among friends. Or it could be part of an experiment in an art project. Sometimes, it can also be to support a cause, like promoting gender equality by showing that clothing doesn't have to be strictly gender - defined. For example, if there is a local campaign about breaking gender norms, he could be participating in it by dressing like a girl.
Another reason might be to understand the other gender better. A boy in a story could dress up like a girl to experience what it's like to walk in their shoes, to face the different expectations and social interactions that girls typically have.
Well, in this fanfiction, it could be that Harry Potter has come across some sort of ancient magic or artifact that has changed him. Maybe this magic has a connection to Tobi's essence or the type of power that Tobi wields. Additionally, he might be trying to protect his friends in a different way. Tobi has his own way of safeguarding what he holds dear, which might seem appealing to Harry in a difficult situation. Or, there could be a prophecy or a hidden message that he has misinterpreted, leading him to believe that acting like Tobi is the right path to take.
Maybe Dumbledore wanted to protect Harry from something specific. By changing his gender, he could hide Harry from certain threats. It could also be a way to teach Harry something, like seeing the wizarding world from a different perspective.
One reason could be a devil fruit power gone wrong. Maybe he ate a fruit that was supposed to give him some new ability but had an unexpected side effect of changing his appearance to that of a girl. Another possibility is that he was hit by some strange magic or curse during an adventure on an island. It could also be the result of a prank by some powerful and mischievous character who has the ability to change appearances.
Well, such a fanfiction would likely rewrite a lot of the original plot. Harry as a girl might face different challenges at Hogwarts. There could be more focus on her dealing with the typical girl - related issues in a magical world, like how she dresses for the Yule Ball in a different way than the original Harry. Her relationship with Draco could also be different. Instead of just being rivals, there could be a more complex, perhaps romantic, undertone to their interactions as she, as a girl, might be seen as more of a threat or an object of interest to Draco in a different sense.