One common reason is Snape's lingering hatred for James Potter. Since Harry looks so much like his father, Snape might take out his old grudges on Harry. He sees James in Harry and can't help but be reminded of the past bullying and his unrequited love for Lily. So, he acts hurtfully towards Harry as a way to get back at James, even if it's not really fair to Harry.
Another reason could be Snape's own insecurities. Maybe he feels threatened by Harry's popularity and natural talent for magic. In the fanfiction world, Snape might see Harry as a rival, not just to his own position at Hogwarts but also in the eyes of the other characters like Dumbledore. So, to assert his own authority and superiority, he hurts Harry through his words and actions.
One possible reason could be his lingering resentment towards James Potter, Harry's father. Snape and James had a long - standing feud during their school days. Since Harry looks a lot like James, Snape might be transferring his old grudges onto Harry.
One common reason could be his love for Lily. Since Harry is Lily's son, Snape might feel an obligation to protect him out of his undying love for her. It's like he's trying to preserve a part of Lily that still exists in the world through Harry.
Another reason might be that Harry is suspected of sneaking around in areas where he's not supposed to be. Snape, as a vigilant teacher, may punish Harry to maintain discipline in the school. He may think Harry is up to something related to the ongoing dangerous situations in the wizarding world and wants to keep him in line.
One possible reason could be that in the fanfiction, the author might explore Snape's complex past with Harry's mother, Lily. Snape's love for Lily could translate into a more nuanced view of Harry. He may see parts of Lily in Harry and that softens his hatred.
In some fanfics, Snape might hurt Harry out of his long - held grudges. Snape was in love with Harry's mother, Lily, and Harry being a reminder of James, who bullied Snape in school, could trigger his negative emotions. Also, Snape's loyalty to Voldemort in the earlier days in some fan - created scenarios might lead him to target Harry, who was seen as a threat to Voldemort's plans.
One possible reason could be Snape's long - standing grudge against James Potter, Harry's father. Since Snape had a difficult past with James, he might project that onto Harry. Another reason could be that Snape is trying to protect Harry in a very strange way. He may think that by rejecting Harry, he is keeping him at a distance from the dangerous wizarding world politics and plots that Snape is often involved in.
In some fanfictions, Snape's impulses towards Harry can be complex. One common portrayal is that his lingering love for Lily extends to a sense of protectiveness over Harry. He might have an impulse to keep Harry safe from the real threats in the wizarding world, even if he doesn't show it openly. This is often a way for fanfiction authors to explore the deeper, unspoken emotions Snape could have had for Harry based on his relationship with Lily.
One possible reason could be that in the fanfic, Snape realizes Harry has more of Lily in him than he initially thought. Harry might show kindness or a particular trait that reminds Snape of Lily, softening his feelings.
In some fanfics, Snape might punish Harry because of his perceived insolence in class. Harry has a rebellious streak, and Snape, being strict, wouldn't tolerate any behavior that he deems disrespectful to his authority as a teacher.
One example could be that Snape secretly helps Harry with his Potions work. He might leave some useful hints or books where Harry can find them. Maybe he gives Harry a potion ingredient that is hard to get when he sees Harry struggling to complete a difficult potion recipe.