It could be that in the fanfiction world, the social structure is so corrupt that stealing from the rich or powerful who are abusing their positions is seen as a heroic act. Harry, being the hero, takes on this role of a 'good thief' to redistribute resources or expose the wrongdoings of those in power.
Maybe he has to steal to help his friends. If a friend is in danger and the only way to get the necessary items to rescue them is through stealing, like stealing a potion ingredient from a restricted area in Hogwarts. In fanfiction, the boundaries of right and wrong can be blurred to create more complex and interesting stories.
In some fanfictions, Harry Potter could potentially be depicted as a 'good thief' in a rather unorthodox sense. For example, if he's stealing something for a noble cause like stealing a powerful artifact from a dark wizard to prevent it from being misused. But in the original canon, he is not a thief.
One reason could be to explore the darker side of human nature. Even though Harry is a hero in the original series, fanfiction writers might want to see how he would be affected if he was pushed to his limits. Another reason is to create a more complex and controversial storyline. It grabs readers' attention as it goes against the normal perception of Harry.
Some fanfiction writers may have a different vision for Harry Potter's character. They might see him as more of a diplomat or a healer within the wizarding world, rather than a fighter. So, they write stories that reflect this different view of the character, where he uses his wits and other skills rather than engaging in physical or magical combat.
One reason could be that the official Harry Potter canon is so extensive and detailed that it overshadows fanfiction. The original story has such a rich world, characters, and plotlines that it becomes the main focus for many fans, leaving less room for fan - created stories.
One reason could be safety. Some fanfiction authors might think that Hogwarts is too dangerous for Harry, so they make him homeschooled to keep him out of harm's way.
One reason could be a reaction to the Ministry of Magic's mismanagement. In the original story, the Ministry often made mistakes and had corrupt elements. In fanfiction, this could lead to Harry becoming an anarchist as a way to oppose such a flawed system.
One possible reason could be his past experiences with some cruel adults, like the Dursleys. They treated him horribly, so in fanfiction, this trauma might make him generally fearful of adults. Another reason could be his experiences with certain authority figures at Hogwarts who may have been strict or unfair, which could be exaggerated in fanfiction to create a fear of adults.
A possible reason could be jealousy. Maybe Harry sees Dumbledore getting all the praise and respect while he is the one doing all the dangerous work. So, he decides to expose some of Dumbledore's flaws to get some of that attention back.
The doctor could be using it as a form of relaxation. Reading fanfiction is a fun way to unwind, and Harry Potter fanfiction offers a wide range of stories within a familiar magical world. It can be an escape from the daily stress of a doctor's work.
One reason could be jealousy. In some fanfictions, if Harry has become very famous and powerful, he might think the Weasleys have different motives for being around him, like using his fame for their own gain. So he starts to lose trust in them.