Maybe in those fanfictions, the author portrays McGonagall in a way that Harry doesn't like. For example, the character might be out of character and act too harshly or differently from how he expects her to be in the original story.
It could be that Harry has a very specific and personal view of Mcgonagall. The fanfictions often add a lot of fictional elements that deviate from his own perception of her. For example, they might make her act out of character in terms of her strictness or fairness, which he doesn't like.
In some fanfictions, Harry might not forgive McGonagall because she may have made a decision that Harry feels was a great betrayal. For example, if she sided with the Ministry when they were unjustly treating him. It could also be that she failed to protect him or someone he cared about in a crucial moment within the fanfiction's plot.
It could also be due to some actions of McGonagall that seemed suspicious to Harry. Perhaps she made a decision regarding Hogwarts or Harry's safety that Harry didn't fully understand at the time. In the complex world of the wizarding school, decisions are often made with many factors in mind, but Harry might not be aware of all of them, leading to his distrust.
McGonagall might notice Harry's distrust and feel hurt or frustrated. She may try to figure out what went wrong but find it difficult as Harry is not forthcoming. Their interactions become less warm and more formal. For example, in class, Harry may be less likely to ask for help from McGonagall, and McGonagall may be more strict with Harry as she doesn't understand his change in attitude.
Yes, there are many. There are a lot of fanfictions that focus on their mentor - mentee relationship in a positive and respectful way. For example, some might explore how McGonagall helps Harry develop his magical skills further or how she offers him guidance during difficult times at Hogwarts. These types of fanfictions stay true to the spirit of the original series and do not involve any inappropriate content.
It could also be that in the fanfiction, McGonagall has shown some form of bias. Maybe she seems to favor certain students or groups over others, and Harry, being the one who always stands up for fairness, starts to distrust her when he notices this behavior. He might feel that she is not as just as he once thought her to be.
Well, in some fanfictions, the author might want to create an alternate scenario where Harry has different relationship dynamics. Maybe they want to explore other pairings for Harry and so they write him as not liking Ginny to deviate from the original storyline.
Maybe Harry thinks fanfiction often distorts the real relationship between him and Ron. He values their genuine friendship and doesn't like the fictional and sometimes over - the - top interpretations in fanfiction.
Maybe in some fanfictions, Dumbledore's character is portrayed in a way that Harry doesn't like. For example, if the author makes Dumbledore overly controlling or having some hidden motives that are against Harry's interests.