Maybe Lucy has some past traumas or insecurities related to love. She could be afraid of getting hurt or losing Natsu as a friend if the relationship doesn't work out. So, she rejects his love to preserve their current friendship.
One possible reason could be that Lucy is too focused on her own goals, like becoming a stronger mage or achieving something specific in the guild. She might see a romantic relationship as a distraction at that moment.
One reason could be that the fanfic focuses on a different pairing for Lucy. So, to make that other relationship more plausible, the author made Lucy not have feelings for Natsu.
Natsu might be initially shocked and sad. He could go off on his own for a while to think about what happened. Since he's a passionate person, the rejection would hit him hard.
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.
He may try to understand Erza's reasons. Natsu, despite his impulsive nature, has a big heart. He might respect Erza's decision and step back, trying to keep their friendship intact. However, he could also be a bit persistent in a non - annoying way. He might keep showing his care for her in a more friendly manner, hoping that over time, she might change her mind.
A possible plot could be that their married life is disrupted by a mysterious force that is causing chaos in the magical world. Natsu and Lucy, being the strong couple they are, set out to find the source of this problem. As they journey, they discover secrets about their own powers and how their union might be the key to saving the world. There could be moments of doubt and fear, but their love for each other always pulls them through.
One possible reason could be Snape's long - held grudges and resentments. He has a complex history with Harry's father, James Potter. Snape might see too much of James in Harry and be unable to separate the two, thus rejecting Harry despite any potential father - son connection. Another reason could be Snape's own self - loathing and his belief that he is not worthy or fit to be a father figure. He may think that he would only bring harm or a bad influence to Harry's life.
One possible plotline could be that Natsu and Lucy go on an adventure to find a special lemon tree that is said to have magical properties. Along the way, they face challenges that make them realize their feelings for each other more deeply. For example, they might have to protect each other from dangerous monsters. As they search for the lemon tree, they share tender moments and by the time they find it, they confess their love under the shade of the tree.
One reason could be for the shock value. Authors might want to create a very different and unexpected scenario to draw readers in. Another could be a misinterpretation of Natsu's passionate and brash nature as something more negative and controlling.
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.
There are a lot of fanfics out there, but ones with sexual content are not in line with the general positive spirit of the Fairy Tail franchise. We should look for fanfics that are more about the growth of the characters, their relationships in a non - sexual way, like how Lucy and Natsu support each other during battles and in their quest for magic.