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.
After Natsu gets angry, the plot might develop with other guild members trying to mediate. Happy might try to calm Natsu down while Gray could be in the middle trying to make both sides see reason. This could create a lot of comical and tense moments in the fanfiction.
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.
In a fanfiction, Natsu might get angry if he was wrongly accused of something. Say, if he was blamed for a crime he didn't commit in the guild or in the magical world at large. He's an honest person, and being falsely accused would be a huge blow to his pride. Another possible reason is if a magic - based experiment went wrong because of someone else's carelessness and it put his friends in danger. Natsu values the safety of his comrades, and such a situation would make his blood boil. Also, if someone were to try and break up the Fairy Tail guild by causing internal disputes, Natsu would be enraged as he loves the unity and spirit of the guild.
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 a misunderstanding during a mission. Maybe Erza's actions were misinterpreted by Natsu, leading to him getting hurt, either physically or emotionally.
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.
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 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 fanfictions, Natsu's anger often has a significant impact on his relationships. His anger towards enemies unites him more closely with his Fairy Tail comrades. They see his passion and are inspired to fight alongside him. However, his anger can also create misunderstandings. For example, if he gets angry at a new member of the guild who made a mistake but was actually trying to help, it could make that person feel unwelcome at first. Overall, his anger is a double - edged sword in his relationships.
One possible reason could be that the Cullens make a decision about her future without consulting her. For example, if they decide she should leave Forks for her own safety without asking her how she feels about it.