Maybe he is new in the place and has not had the chance to make friends yet.
Perhaps he has some special powers or abilities that others are jealous of or afraid of. In the world of fanfiction, this can often be a reason for isolation. His powers might be too strong or uncontrollable, making people around him keep their distance. Also, he might be from a different social group or background that causes a divide between him and potential friends.
One possible reason could be a divergence in their goals. Maybe in the fanfiction, Harry has a different approach to fighting Voldemort and Ron doesn't agree with it, leading to a rift. For example, Harry might want to take a more aggressive and risky approach while Ron prefers a more cautious one. This difference in ideology can drive them apart.
One possible storyline could be Harry making friends with students from different houses at Hogwarts. For example, he befriends a shy Ravenclaw who helps him with his studies in a unique way. Another could be Harry befriending magical creatures, like a friendly centaur who gives him advice on dealing with the Forbidden Forest.
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.
One reason could be extreme stress. In fanfiction, Harry might be facing a new and overwhelming threat, like a powerful dark wizard that even his skills can't easily handle. The pressure of having to protect his friends and the wizarding world could cause him to faint.
One possible reason could be a misunderstanding. Maybe a miscommunication led to a rift between them. For example, if Harry overheard Ron saying something that was taken out of context, it could start the breakdown of their friendship.
One possible reason could be extreme depression. Maybe in the fanfiction's plot, Harry has faced so many losses and hardships that he feels there is no way out. For example, if all his friends turn against him or he fails to achieve his goals repeatedly.
Well, in some versions of the 'Dursleys abandon Harry fanfiction', they abandon him due to social pressure. They are worried about what their neighbors or friends might think if they find out about Harry's true identity. Also, Petunia may have some deep - seated jealousy towards her sister Lily for being a witch, and this negative feeling extends to Harry, leading them to abandon him.
Another reason might be some kind of external influence, like a powerful dark magic that clouds Harry's judgment. However, this is often a stretch from the original concept of the characters. Fanfiction writers sometimes like to explore extreme scenarios to add drama to the story.
One possible reason could be a major disagreement with the Gryffindor house leadership or some of the students. Maybe he feels that they are not living up to the values he thought they held. Another reason might be that he discovers some dark secret within Gryffindor that makes him uncomfortable and he decides to leave.
One reason could be that in this fanfiction's unique plot, there is a prophecy or some ancient magic that has been misinterpreted. This misinterpretation leads Harry to think that Muggle - borns are somehow linked to an impending doom for the wizarding world. So, as a result, he harbors hatred towards them. It could also be that there has been a series of unfortunate events that he wrongly blames on Muggle - borns, like a magical accident that he believes was caused by their 'uncontrolled' magic.