Perhaps in the fanfiction, Erza rejects Natsu because she has some past traumas or secrets that she doesn't want to involve Natsu in. She may think that by rejecting him, she is protecting him from getting hurt. Also, it could be that there are other suitors for Erza who she is considering more seriously at the time. These suitors might seem more stable or have more to offer in terms of status or power within the guild.
One possible reason could be that Erza is focused on her own goals and career in the Fairy Tail guild. She might see a relationship as a distraction at that moment. For example, she is always striving to be stronger and take on more challenging missions. Another reason could be that she may not view Natsu in a romantic way but more as a close friend and guildmate. Maybe she has a different ideal type in mind, someone more calm and studious rather than Natsu's brash and impulsive nature.
It could be due to differences in their magical abilities. Erza might be worried that a relationship with Natsu could lead to dangerous magical combinations or problems. She is very disciplined with her magic use, while Natsu is more instinctive. This difference in approach to magic could be a factor in her rejection. Also, she may be concerned about how their relationship could affect the dynamics within the guild, especially if it causes jealousy or division among the members.
Natsu might be initially shocked and confused. He's not the type to expect rejection, so he could be at a loss for words at first. For instance, he might just stand there with his mouth open, not believing what he just heard.
One possible reason could be that Erza might have taken on a dangerous mission alone without informing Natsu. Natsu is protective of his friends and would get angry if he thought Erza was putting herself in unnecessary danger.
One possible scenario could be that Natsu confesses his love to a character, perhaps Lucy, and she rejects him because she doesn't see him in that way. She might think of him only as a good friend. Another scenario could be that Natsu is rejected in a job or a quest application within the fanfiction world. Maybe the guild he wants to join or the person in charge of a certain task doesn't believe he has the skills or the right attitude.
One possible reason could be a misunderstanding during a mission. Maybe Erza's actions were misinterpreted by Natsu, leading to him getting hurt, either physically or emotionally.
It could be that Erza scolds Natsu too harshly for something he did wrong. Natsu, who might be more sensitive than he seems, could end up crying from the severity of her words.
Sometimes, it's just a way to shock the audience. Fanfiction authors often like to take risks and do unexpected things to their beloved characters. Torturing Natsu can be a very unexpected and extreme move that can leave a lasting impression on the readers, whether they like it or not.
One possible reason could be that Hermione is too focused on her studies and career goals in the wizarding world. She might see a relationship with Harry as a distraction. Another reason could be that she has different values when it comes to love. Maybe she wants someone more like her, who is more into books and intellectual pursuits all the time, while Harry is more action - oriented.
One possible reason could be a misunderstanding within the guild. Maybe some false information was spread about them, leading others in Fairy Tail to turn against them. Another reason might be a power struggle. If Natsu and Erza were becoming too powerful, some jealous members could have conspired to make it seem like they were betraying the guild. Or perhaps an outside force manipulated the situation to drive a wedge between them and the rest of Fairy Tail.
One possible reason could be a misunderstanding. Maybe Erza misinterprets Natsu's actions as being reckless and endangering the guild, so she takes harsh measures to 'teach him a lesson'.
There could be such a fanfiction. Fanfiction writers often come up with all kinds of creative and sometimes out - of - the - box ideas. However, I haven't personally come across one where Natsu gets Erza pregnant. It might be a rather niche concept within the Fairy Tail fanfiction community, as most fanfictions tend to focus on different aspects of the characters' relationships, like friendship, adventure - related partnerships, or other forms of romantic development.