One possible reason could be a misunderstanding. Maybe Harry did something that was misinterpreted by his mate. For example, if he was seen talking to another character in a friendly way, his mate might have thought he was being unfaithful. Another reason could be differences in their long - term goals. If Harry's mate has a very different vision for their future, like wanting to live a quiet life while Harry is more adventurous, this could lead to rejection.
The mate might have some personal issues that have nothing to do with Harry directly. Maybe they are dealing with their own insecurities or past traumas that make them feel unworthy of a relationship. So, instead of dragging Harry into their mess, they choose to reject him.
It could be due to external pressure. In the fanfic world, there might be some outside forces, like an evil character threatening Harry's mate to stay away from him. Or perhaps there are family or social expectations that conflict with their relationship. For instance, if Harry is from a different social class or magical group, his mate's family might oppose the relationship, causing the mate to reject him.
One reason could be differences in values. For instance, if Draco's mate has strong beliefs in equality and Draco has some traditional pure - blood supremacist ideas left from his family upbringing, this could lead to rejection.
One possible reason could be that he wants to break away from the pre - determined path. In fanfic, the author might want to explore an alternative version of Harry who is tired of the constant danger and expectations at Hogwarts. He may feel that staying there will only lead to more battles and losses, so he rejects it to seek a normal life.
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.
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.
One possible reason could be that Harry is too focused on his fight against Voldemort and doesn't want to put Ginny in danger. He might think that being with her would make her a target for his enemies.
One possible reason could be that Erza is too focused on her own goals and duties. She has always been committed to protecting the guild and her comrades. Maybe she thinks that getting involved in a romantic relationship with Natsu would distract her from these important tasks.
In some fanfics, they might abandon Harry because they are under a dark spell. This spell could make them lose their love for him and force them to leave. Another reason could be that they are blackmailed by some powerful dark forces. These forces may threaten their lives if they don't abandon Harry.
She could be rejecting Harry's proposal because she's still dealing with past traumas or unresolved issues. For example, she might be worried about how the public would view their relationship, especially given Harry's status as 'the boy who lived'. She also might not be ready for such a big commitment due to the instability in the wizarding world at that time. There could be a lot of political and social unrest, and she might feel that getting engaged is not a priority in such a situation.
One possible reason could be that in the fanfic, Hinata has different personal goals or ambitions that she feels would be compromised if she were to be with Naruto. For example, she might be more focused on her family duties or her own ninja training and career, and she believes that a relationship with Naruto would distract her from those things.
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.