A common scenario is when Naruto is given a special mission or task that Sasuke feels he should have been chosen for. Sasuke's pride and his competitive nature with Naruto make him jealous. He might think that Naruto is being favored over him.
In some fanfictions, if Naruto starts to gain a lot of popularity in the village for some heroic act, Sasuke may get jealous. He may feel that his own contributions are being overlooked. His jealousy can lead to him distancing himself from Naruto and the village, which then adds more drama to the story as Naruto may try to reach out to him.
A common scenario is when Naruto is praised by someone else, like another ninja in the village. Sasuke may get jealous because he wants to be the one Naruto looks up to or the one whose opinion matters most to Naruto. Another scenario could be when Naruto makes a new friend, and Sasuke sees them spending a lot of time together. He might feel left out and jealous.
A common scenario is when there's a new character introduced. This new person starts getting a lot of attention from Blake's significant other or close friends. For instance, if Blake has a romantic partner, and a new person joins their group who is very charming and friendly, Blake might start to feel jealous as she sees her partner enjoying the new person's company.
In many Sasuke - gets - jealous - of - Naruto fanfictions, it often stems from Sasuke's insecurities. He may be jealous of Naruto's natural ability to make friends easily. Naruto has a charisma that draws people in, and Sasuke, who has always been more of an loner, may feel left out or envious. For instance, if Naruto is praised for his skills in a particular area, Sasuke might feel jealous as he also wants that kind of recognition but is too proud to show it in a normal way.
One common scenario is when a new character is introduced. If this new character shows an interest in Starfire, Robin gets all riled up. He starts doubting himself and thinking that Starfire might like this new person more.
One possible scenario could be a power imbalance. Maybe Sasuke discovers some new, extremely powerful jutsu that Naruto has no defense against. For example, a jutsu that can nullify Naruto's chakra or attack his spirit directly. Another possibility is a miscommunication or a breakdown in their relationship. If Sasuke believes Naruto has betrayed him in some way, his anger could lead him to kill Naruto. However, this goes against the core of their bond in the original series.
One common scenario could be when other female characters in the fanfiction show a strong interest in Naruto. For example, if a new female character is introduced and she is constantly flirting with Naruto, Sakura might get jealous. Another situation could be when Naruto gives more attention to someone else's skills or achievements over hers in the fan - made story. It might make her feel left out or less important to him.
Often in these fanfictions, Sasuke gets jealous when Sakura is training with someone else who is really good at a particular skill. Let's say she's training with Rock Lee to improve her taijutsu. Sasuke might see how dedicated Lee is to helping Sakura and get jealous. He may think that he should be the one helping her. And then he might try to interrupt their training sessions or show off his own skills in front of them to prove that he's better, all because of his jealousy.
In the fanfiction, perhaps there's a new recruit in the Joker's gang who is very talented and starts getting a lot of attention. The Joker, being used to being the center of attention, gets jealous. He might start giving the new recruit impossible tasks to do in order to undermine them and regain the spotlight.
One common scenario could be when another guy shows interest in Sailor Moon. Darien might see this guy being overly friendly or flirting with her at a party or in public. For example, if a new hero from another planet arrives and is immediately drawn to Sailor Moon's beauty and power. Darien would start to feel jealous, his eyes narrowing as he watches them interact. He might become more possessive, trying to keep Sailor Moon closer to him and away from the new guy.
One common scenario is when another girl shows interest in Percy. For example, if a new demigod arrives at Camp Half - Blood and she's constantly around Percy, Annabeth might get jealous. She could start acting cold towards Percy or be overly critical of the new girl. Another situation could be if Percy has to go on a quest with a female partner and they seem to be getting along really well. Annabeth might worry that Percy will develop feelings for this other girl.