It could be the Death Eaters. Even if they are on the 'bad side', they might see an opportunity to disrupt Harry's life and weaken him. By forcing him to reject his mate, they can create emotional turmoil for Harry, which would make him less focused on fighting against them.
One possible reason could be some sort of external threat. Maybe there's a dark force that wants to use his mate against him, so to protect her, he's forced to reject her. For example, if there are dark wizards who plan to capture his mate and use her as a bait to get to Harry, he might think rejecting her would keep her out of danger.
It could be Hermione. In some alternate interpretations, there could be a situation where Hermione, while trying out a new and powerful magic, accidentally hurts Harry. Their friendship is put to the test as Harry has to deal with the pain and Hermione has to come to terms with what she's done.
Maybe it's Ron. Suppose Ron gets jealous of Harry's fame and power and, in a moment of weakness and envy, decides to abandon Harry to die. This would create a lot of drama as Ron and Harry have been friends for a long time, and such a betrayal would be a huge shock to the story's readers.
Perhaps Severus's own self - image is one of solitude and self - loathing. He has spent so much of his life in the shadows, used to being alone and unloved. The idea of having a mate, especially someone like Harry who represents so much hope and light, is so foreign to him that he rejects Harry out of fear of the unknown and a sense of not belonging in a relationship.
In Harry Potter fanfiction, Hermione could potentially be Fenrir's mate in some very creative and out - of - the - box interpretations. Some fanfic writers might see her as a character who could be forced into an unexpected relationship with Fenrir, perhaps as part of a dark plot twist where Fenrir captures her and they develop a strange, warped relationship over time.
In this fanfiction, it might be that his mate has some hidden secrets. When these secrets are revealed, it causes a situation where Harry gets hurt. For example, his mate might be related to an enemy in a way Harry didn't know. This revelation leads to a conflict and Harry ends up getting injured in the process. Harry then has to decide whether to forgive his mate or not based on the circumstances.
Well, it could be a completely new character created specifically for this fanfiction. Maybe an elf or a human from Middle - Earth who has some sort of special connection with Harry.
One possible plot could be that Harry's mate is influenced by dark forces. Maybe they are threatened or corrupted, which leads them to abandon Harry. Harry then has to find his own way to survive. He might discover new powers or allies in the process, and in the end, seek revenge on those who made his mate turn against him.
One reason could be that his mate has different life goals. Maybe she is more focused on her own magical career and doesn't see a relationship with Harry fitting into that at the moment. Another possibility is that there are misunderstandings between them. For example, Harry might have been too involved in some dangerous adventures and his mate thought he didn't value their relationship enough. Also, it could be due to external pressures, like her family or friends not approving of Harry for some reason.
It could also be due to differences in their personalities or goals. If Harry is very adventurous and his mate is more cautious, they might not see eye - to - eye on certain important matters. And this fundamental difference could lead to the mate rejecting Harry.