Perhaps it's related to Harry's association with his father, James Potter. Snape had a long - standing grudge against James, and seeing Harry might bring back those bitter memories. So, whenever Harry does something that reminds Snape of James, like being overly confident or a bit of a troublemaker, he yells at him.
Harry might become more determined to prove Snape wrong. He's a stubborn character, and Snape's yelling could fuel his desire to show that he's not the misbehaving student Snape seems to think he is.
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.
Snape might initially be taken aback. He's used to students being intimidated by him, so Hermione's outburst could shock him into momentary silence. Then he might retaliate with his usual sarcasm, trying to put her in her place.
In some fanfictions, Snape might yell at Draco because he expects more from him. Draco is from a prominent pure - blood family, and Snape, being strict, wants Draco to uphold certain standards. Maybe Draco made a mistake in potions class, or he showed disrespect towards other students. Snape's yelling could also be a way to protect Draco from getting into more serious trouble in the dangerous Hogwarts environment.
Well, in some fanfics, Snape's role in the wizarding world politics or his secret allegiances could be a factor. Harry might be aware that Snape has connections to different factions, and he doesn't fully understand where Snape's loyalties lie. This uncertainty can create fear. Also, Snape's teaching style, which can be quite intense and unforgiving, might make Harry feel constantly on edge around him.
At first, Harry is likely to be angry. He might feel that Snape is being unfair and targeting him. Harry has a strong sense of justice, and when he believes he's being wrongly punished, he'll be resentful.
In some fanfics, at first it gets worse. Harry might start to fear or hate Snape even more after being hurt. But as the story progresses, there could be a turning point. Maybe Harry discovers Snape's real motives behind the hurt, like if Snape was trying to protect him from a greater danger in a very roundabout way. This could lead to a begrudging respect or even an odd sort of friendship in the end.
At first, Harry might be very suspicious of Snape's kindness. After all, in the original story, they had a rather tense relationship. But as Snape continues to be kind, Harry starts to see him in a new light. He begins to trust Snape more and more.
In some fanfictions, Harry might defend Snape because he comes to understand Snape's complex past. Snape was in love with Harry's mother, Lily, and his actions, though often harsh towards Harry, were sometimes motivated by his love for her and his role as a double agent against Voldemort. Harry, being a character who grows in understanding, could defend Snape once he learns these truths.
Snape terrifies Harry in fanfiction perhaps because of his strict and often cold demeanor. In the original series, Snape was harsh towards Harry, always seemingly looking for reasons to deduct points or give him detention. In fanfiction, this could be exaggerated. His dark appearance, with his long black robes and sallow complexion, also adds to the menacing aura.