Perhaps the secret is that Lucy has a unique magical item that is very powerful and dangerous. Natsu keeps it a secret because he doesn't want other mages in Fairy Tail to get greedy or misuse it. This item could be something that was passed down in Lucy's family and she doesn't fully understand its potential yet.
He might be protecting her. Natsu cares a great deal about Lucy. If the secret got out, it could put her in danger, either from outside threats or from within the guild if they misinterpret the situation.
One possible reason could be to seek new adventures. They might feel they've reached a point in Fairy Tail where they've done all they can and want to explore new magic and challenges elsewhere. For example, they could have heard of a far - off land with a different type of magic that they are eager to learn about.
A powerful demon from another dimension could be the main villain. This demon has noticed Natsu's growing power and sees him as a threat. The demon has the ability to create illusions and false memories, which could be used to drive a wedge between Natsu and the Fairy Tail guild. He works in secret, slowly weakening Natsu's mental state before making a big move.
Perhaps Natsu is under the influence of a powerful and dark magic that makes him lose control of himself and accidentally hurt Lucy. It could also be that Natsu is trying to protect Lucy from something but his actions end up causing her harm instead. Like if he tries to keep her away from a dangerous situation by pushing her too hard and she gets injured.
In a fanfiction, there could be a situation where Lucy is forced to side with a group that Natsu is against. This would make Natsu see her as an enemy. Or perhaps there is a prophecy that Lucy is involved in that Natsu misinterprets as something bad. He might think she is a threat to the guild or his friends. Also, if there is a love triangle situation where Lucy chooses someone else over Natsu in a way that really hurts his feelings, that could be a reason for him to start hating her.
Perhaps Natsu has a secret friendship with a character from another guild. They meet in secret and exchange valuable information or magic items. This could add an interesting layer to his character. Also, what if Natsu secretly has a connection to a powerful and ancient dragon that no one in the guild knows about? It could lead to some epic storylines.
One possible plotline could be that Lucy uses her charm and intelligence to plan elaborate schemes to get Natsu's attention. For example, she might invite him on special adventures that are just for the two of them, where she can show off her skills and bond with him more deeply. Another could be that she gives Natsu special gifts that are meaningful to him, like a scarf she made herself which has some magical protection. Or perhaps she starts dressing in a way that she knows Natsu will find appealing, not in a vulgar way but in a way that highlights her beauty and confidence.
Well, Natsu might protect Lucy by using his dragon - like senses. In the Fairy Tail world, his enhanced senses can detect danger before it reaches Lucy. So, he could quickly react and pull her out of harm's way. Also, his loyalty to Lucy and the Fairy Tail guild drives him to protect her. He would not let any harm come to her, whether it's from other guilds who might have a grudge against Fairy Tail or from individual villains in the fanfiction. He'll be there to fight for her safety.
Well, one possibility could be that it's a really innocent situation. Maybe they got into some kind of magical mishap that made them dirty and the only available place to clean up was a shared shower area. But they're both really embarrassed and trying to be respectful of each other's space while also dealing with the comical aspect of the situation.
Well, if Natsu asks Lucy out in fanfiction, it could change the whole dynamic of the Fairy Tail group. Their friends would probably be super supportive, especially Happy. They could start exploring new places not just for quests but also for romantic getaways. And Lucy might write a new story about their relationship in her diary.