In fanfiction, there are numerous interpretations for Harry having scars on his back. It could be a symbol of his past traumas. For example, he could have been bullied at Hogwarts in a more extreme way that left physical scars. Or it could be related to his family history. Maybe there was some dark family secret that involved a ritual which scarred his back. Additionally, it could be from a forbidden magic he tried to learn in order to defeat Voldemort once and for all, and the magic had unforeseen consequences on his body.
Perhaps there's a plot against Harry by some dark forces, and the Potters believe that only they can protect him properly. They might think that the people who have been taking care of Harry so far are not equipped to handle the new and more dangerous threats. Also, it could be that the afterlife in the wizarding world has some rules or opportunities that allow them to try and get Harry back into their family fold. They could have been given a second chance, so to speak, and are eager to take it.
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 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 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.
It might be related to power dynamics. If in the fanfiction, James has a lot of power and misuses it in a way that affects Harry or those around him, Harry would hate him. For example, if James uses his influence to push through unfair rules or to marginalize certain groups that Harry supports. Also, if the fanfiction changes James's personality to be more self - centered and less empathetic, Harry would likely dislike him.
Another possibility is that he doesn't like how some fanfictions over - sexualize the characters. In some cases, fanfiction can turn into something inappropriate, and Harry, who has a more pure view of his friends and the wizarding world in general, would be disgusted by this. He wants to preserve the integrity of the relationships and the world as he knows it.
Maybe in that fanfiction, McGonagall has some actions that Harry misinterprets. For example, she might be too strict with him in a way that he can't understand at that moment.
One reason could be that Ron abandons Harry when he needs him the most. Like in a dangerous situation, if Ron runs away leaving Harry alone to face the threat, Harry would find it hard to forgive such cowardice.
One reason could be their friendship. Harry values Luna as a friend and doesn't want to see her hurt.
In some fanfictions, Ginny may reject Harry because she wants to focus on her own personal growth. She might feel that a relationship with Harry at that time would hold her back from achieving her own goals, like becoming a great Quidditch player or an accomplished witch in her own right. So, she decides to reject him to pursue her own dreams first.