Perhaps in the fanfiction, his friends failed to recognize his efforts. Harry might have worked really hard on something, like preparing for a big event or helping them in some way, but his friends didn't seem to appreciate it at all. This lack of recognition could easily make him disappointed in them.
Maybe his friends in the fanfiction betrayed his trust. For example, they could have broken a promise to him, like not showing up when he really needed them. This would make Harry feel let down.
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.
There could be a situation where McGonagall withholds important information from Harry regarding his family or his destiny. In the world of Harry Potter, knowledge is power, and Harry has always been in search of the truth. If McGonagall kept something crucial from him, it could create a rift that leads to him not forgiving her.
A Quidditch accident could be a start. If Draco were to get seriously injured during a Quidditch match and Harry were the first to help him, Draco might feel gratitude towards Harry. This newfound feeling could break down the walls between them. After that, they might start having conversations in the hospital wing, slowly realizing they have more in common than they thought.
One possible plotline could be Harry feeling extremely isolated and trying to find new friends outside of his usual circle. Maybe he starts to focus more on his studies and discovers new magical abilities on his own.
In some fanfictions, Naruto might be hated by his friends due to a misunderstanding. For example, if there's a false rumor spread about him being a traitor. Another reason could be his sudden increase in power that makes his friends jealous. Maybe he accidentally harms one of them during a training session and they hold a grudge.
Perhaps the father is under the influence of some external force. In fanfiction, it could be that he is being blackmailed or controlled by a dark wizard. As a result, he has to reject Harry against his true feelings. Also, it could be that the father has made some big mistakes in his life and he doesn't want Harry to be associated with him because he thinks it will bring harm to Harry.
One possible consequence could be that Harry becomes withdrawn and distrustful. He might have difficulty forming relationships with others, always fearing that they will also be cruel like his parents. He could also develop low self - esteem, constantly believing that he is unworthy of love and care because of how his parents treated him.
One possible event could be that the owl met these 'friends' during a food shortage. They pretended to be friendly to get a share of the owl's food stash.
In some fanfictions, it could be that Harry discovers a dark secret about the wizarding world that his friends don't believe or are too afraid to accept. His friends, thinking he has gone mad, side with the established order. They then betray Harry by reporting his so - called 'crazy ideas' to the Ministry of Magic, leading to Harry being hunted.