One possible reason could be Fleur's unique charm. She is part - Veela, which gives her an alluring quality that might make Harry feel insecure. Another reason might be her skills. If in the fanfiction she is shown to be extremely talented in magic, especially in areas where Harry struggles, it could trigger his jealousy.
Perhaps in the fanfiction Fleur has a closer relationship with someone Harry cares about, like Ron or Hermione. If she is always around them and they seem to prefer her company, Harry could become jealous. Also, if Fleur represents a kind of 'ideal' in the wizarding world that Harry feels he can't measure up to, that would also lead to jealousy.
One possible reason could be that Harry admires Bill's adventurous past and Fleur's beauty. He might feel a bit envious because Bill has had these exciting experiences and Fleur seems so alluring. Also, Harry might be jealous of their relationship as he himself has had some difficulties in his own romantic pursuits.
One possible reason could be that Dean is better at something that Harry secretly wishes he was good at. For example, if Dean is really popular among their peers and Harry feels overshadowed. Another reason might be related to a girl they both like. Maybe the girl shows more interest in Dean, and Harry can't help but feel jealous.
His jealousy might make him a bit withdrawn from Bill. He could be less friendly and more distant, which could cause some tension between them. As for his relationship with Fleur, he might act a bit awkward around her, not really knowing how to handle his feelings.
It could be that Bella has a love interest that Lauren also fancies. In fanfiction, love triangles are common. If the object of affection shows more interest in Bella, it would make Lauren jealous. For example, if there's a guy both of them like, and he always chooses Bella over Lauren.
In some fanfictions, it could be due to a prophecy or a magical bond that they discover. They might feel that the only way to fulfill the prophecy or strengthen the bond is by getting married secretly. Another aspect could be the political situation in the wizarding world. There could be political factions that would oppose their union for various reasons, such as Harry being seen as a symbol of a certain side and Fleur being from a different background. So, to avoid political turmoil or to keep their relationship safe from political manipulation, they choose to marry secretly.
His jealousy could also show in his magic. Maybe he tries too hard to outperform Fleur in magic tasks or challenges. He might be constantly comparing himself to her and pushing himself to the limit just to prove that he is better, which is a sign of his jealousy.
One possible reason could be that Luke has certain skills or abilities that Percy admires and feels inferior about. Maybe Luke is better at combat or has more knowledge in a particular area, making Percy jealous.
In the 'Judy Gets Jealous of Nick Fanfiction', Judy might be jealous because the fanfiction could be highlighting Nick's charm and charisma in a way that overshadows her. Fanfiction often exaggerates certain traits of characters. If Nick is shown as being the center of attention all the time in the fanfiction, Judy, who is used to being a strong and important character herself, may feel envious. She might also be jealous if the fanfiction creates a special bond between Nick and another character that she feels should be with her instead.
One possible reason could be Percy's success and popularity. In the fanfiction, Percy might be achieving great things, getting recognition, and Luke, feeling overshadowed, becomes jealous. For example, if Percy is constantly saving the day and being praised while Luke is struggling in the background, that would breed jealousy.
Perhaps in the fanfiction, Judy starts to spend more time with a new character. Nick, who has a deep bond with Judy, sees this new relationship as a threat to their friendship or partnership, and jealousy creeps in. For example, if Judy and the new character are working on a special project together that Nick isn't part of, it could trigger his jealousy.