One possible reason could be that Snape shows a different side to Harry in the fanfiction. Maybe he protects Harry secretly from some hidden threats that others are unaware of, which makes Harry start to trust him.
In the fanfiction, it could be that Snape is the only one who truly believes in Harry's capabilities in a unique way. He might see potential in Harry that others overlook. And he could be constantly guiding Harry in a more positive and understanding way compared to other characters. This special treatment from Snape could cause Harry to trust him.
Perhaps in this fanfiction, Snape shows a side of himself that is completely different from what Harry has known. He might be more open and honest with Harry, sharing secrets that make Harry believe he can be trusted. Like Snape might disclose some long - held information about Harry's parents that only he knew, which makes Harry think Snape has been on his side all along.
Perhaps in this fanfiction, Snape has proven himself to Harry in some extraordinary way. Maybe he has saved Harry's life multiple times or has shown his true loyalties that were hidden in the original story. It could also be that Snape has some special connection with Harry that makes Harry trust him unconditionally.
In this fanfiction, their relationship could develop in a rather unexpected way. Maybe Snape initially approaches Harry for his own reasons, but as time goes on, a real bond forms. Harry, who has always been in need of someone to trust, finds in Snape a kind of stability. Snape, on the other hand, might change his attitude towards Harry completely. He might stop being so cold and distant and instead become a warm and reliable figure for Harry. This transformation in their relationship is likely to be a central part of the fanfiction's plot.
A plotline could be that Snape is framed for a crime he didn't commit, and Harry, who trusts him, is the only one who believes in his innocence. Harry then sets out to prove Snape's innocence, facing many obstacles and enemies in the process. This would be a great twist on the typical relationship between them in the original series.
One reason could be that Harry sees Snape's Patronus, which is the same as his mother's. This could be a strong indication of Snape's true feelings and make Harry trust him.
As the story progresses, they might start to work together more closely. Snape could be teaching Harry some advanced magic that no one else knows, and through this process, they build a bond. Harry begins to understand Snape's past better and realizes that they have more in common than he thought, like their love for Lily Potter in different ways.
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.
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 possible reason could be that Harry is afraid of Snape's intimidating presence. Snape has always been strict and rather unfriendly towards Harry in the original series, so in the fanfiction, this fear might be amplified, causing Harry to run. Another reason could be that Harry has stumbled upon something he shouldn't have seen that involves Snape, and he's running to avoid getting caught.
One possible reason could be Snape's sense of duty to protect Hogwarts students in general. Even though he dislikes Harry, he might put aside personal feelings to ensure the safety of a student in a dangerous tournament like the Triwizard Tournament. Another reason could be Dumbledore's influence. Dumbledore trusts Snape and may have asked him to keep an eye on Harry during this event.