One possible reason could be jealousy. Maybe Harry has achieved something great and his friends can't handle his success. For example, if he gets a very important role in a magical competition, his friends might feel overshadowed. Another reason could be a misunderstanding. Harry might be wrongly accused of something, like being in league with the dark side, and his friends, in their fear or haste, abandon him without giving him a chance to explain.
Perhaps it's due to external influence. A powerful and malevolent force could be manipulating his friends' minds. This force might be spreading false rumors about Harry to drive a wedge between them. It could also be that his friends have their own personal issues to deal with. For instance, one of them might have family problems that require their full attention, and in the process, they unknowingly neglect and eventually abandon Harry.
In some cases, it could be a test of loyalty. The wizarding world might be facing a hidden threat, and Harry's friends are made to believe that abandoning him is for the greater good. However, this is a cruel test that leaves Harry feeling betrayed. It's also possible that over time, his friends' values have changed. They might have started to pursue different goals in life that don't align with Harry's, leading to their separation.
One reason could be a misunderstanding. Maybe there was a miscommunication between Harry and his friends, and he felt so hurt or angry that he decided to leave. For example, if his friends wrongly accused him of something he didn't do.
Maybe his friends betrayed his secrets. In many fanfictions, a friend might accidentally or deliberately let out something Harry had confided in them, like his fears about Voldemort or his feelings for a certain girl. This could shatter his trust.
One reason could be jealousy. Maybe Harry has achieved something great like getting a special magical power or a high honor in their magical world, and his friends feel envious. Another reason might be a misunderstanding. For example, Harry could have been wrongly accused of something, and his friends believe the false rumors without giving him a chance to explain.
Another reason might be jealousy. If Harry suddenly gains more power or attention in the fanfiction world, his friends could become jealous. For example, if he discovers a new and powerful magic spell, and all the focus in the magical community is on him, his friends might start to feel overshadowed and distance themselves from him.
Well, sometimes in fanfiction, the author wants to explore the theme of loneliness. So they create a scenario where Naruto's friends abandon him because they are influenced by the village's elders. The elders might spread false rumors about Naruto to maintain their control. His friends, being young and naive, believe these rumors and turn their backs on Naruto, which then leads to Naruto's journey of self - discovery and growth in solitude.
One reason could be a misunderstanding. Maybe Harry said or did something that his friends misinterpreted. For example, if he was overheard making a comment that seemed critical of them but was actually taken out of context.
One possible reason could be to protect them. In many fanfictions, Harry might believe that by distancing himself from his friends, he can keep them out of danger. For example, if there is a new dark threat targeting him specifically, he may think that the enemy will use his friends against him.
One reason could be to escape the lingering dark magic and the constant reminders of their past battles. Britain, especially Hogwarts, has seen so much turmoil that they might feel the need for a fresh start. Another reason could be to explore the wider wizarding world. They've been mostly centered around Britain in the original stories, and in fanfiction, they could leave to discover new magical cultures and spells in other parts of the world.
Well, one reason could be jealousy. Maybe his friends envy his fame or special abilities. Another could be misinformation. If they are fed false information about Harry, they might turn against him. For example, if someone convinces them that Harry is secretly working for the enemy.
It could also be due to a choice Harry makes that his friends don't agree with. Suppose Harry decides to take on a very dangerous mission alone to protect his friends, but they see it as him not trusting them. This could lead to a rift and eventually him losing their friendship.