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.
Well, in some fanfictions, if Naruto is paired with another character in a potential romantic way, that would surely make Sakura jealous. She has deep - seated feelings for Naruto, so seeing him with someone else, even in a fictional scenario created by fans, can trigger her jealousy. Also, if Naruto spends a great deal of time with another character on a mission or in training in the fanfiction and seems to neglect Sakura, she may become envious.