Another possible reason is to give Harry a chance to grow independently. Without the influence of his friends all the time, he can develop his own skills and beliefs at his own pace. He can face challenges alone and learn from them in a more solitary way, which is a different kind of character development compared to when he's always with Ron and Hermione.
One reason could be to explore a different side of Harry's character. In the original books, he's very much a social character in some ways, but by having him keep to himself, the author can show his more introverted and introspective side.
Authors might write such a fanfiction to create a sense of mystery. If Harry is keeping to himself, other characters in the story will be curious about what he's up to. This can add an element of intrigue and drive the plot forward. For example, the other Hogwarts students might start to wonder if Harry is planning something big on his own.
Maybe the author wants to explore different relationship dynamics. By having Harry ignore his friends, it creates tension and conflict which can make for an interesting story. It could also be a way to show how fragile their friendship is in certain situations.
One reason could be to explore an alternate path for Hermione's character. By having her refuse the prefect position, it shows a different side of her independence and her ability to make choices that go against the norm in the Hogwarts hierarchy.
One popular 'Harry keeps to himself' fanfiction could be 'The Silent Gryffindor'. In this story, Harry's decision to keep to himself is due to the overwhelming attention he gets after the events with Voldemort. He withdraws into himself to find some peace and self - discovery.
Hermione would be a likely helper. She's always full of knowledge. She could research about simulacrums in the library and find some old spells or precautions that Harry should take while creating or handling it.
Another challenge is that without a proper teacher, he may not fully understand the nuances of magic. Magic in the Harry Potter world has a lot of rules and ethics. Harry might accidentally break some of these rules while trying to teach himself. Also, he may not be aware of the proper pronunciation or wand movements for some spells, which could lead to ineffective casting.
For creativity. It's a unique twist on the original story. It allows the author to explore different relationships and how characters would react if Harry had a different gender presentation.
For the sake of creating a unique plot. The 'Harry Potter' series has a well - established story, and by changing such a fundamental relationship as Harry and Dumbledore's, the author can draw in readers who are interested in seeing a fresh take on the familiar world. It can also open up new storylines that might not have been possible in the original series, like how Hogwarts functions without Harry having Dumbledore as a guiding figure.
Maybe Hagrid will teach Harry all kinds of magical creatures in that summer. He could introduce Harry to the hippogriffs, how to approach them safely and gain their trust.
It could be due to the trauma he has experienced. Losing friends and facing Voldemort multiple times has surely left deep scars. Isolating himself might be his way of dealing with the post - trauma stress. He needs time alone to heal and come to terms with everything that has happened.
Well, in fanfiction, Harry may be possessive of Hermione due to his own insecurities. He has lost many people he cares about, like his parents. Hermione has become an important part of his life, and he's afraid of losing her too. His possessiveness could be a defense mechanism. Also, the fanfiction writers might want to create a more intense and complex relationship dynamic between them. By making Harry possessive, they add a layer of drama and conflict. For example, it can create tension between Hermione and other characters who might misunderstand Harry's possessive behavior.