It could be that she secretly likes the smell of it, which is a bit of a personal and intimate reason. In the context of the fanfic, it might also be a symbol of their relationship, and by stealing it, she's making a statement about how she feels towards him, whether she realizes it fully or not.
He could also playfully try to get it back, leading to a bit of a chase or a fun exchange between them. This would add some light - heartedness and interaction to the fanfic.
One possible reason could be a misunderstanding. Maybe Percy did something that Annabeth misinterpreted, like making a decision during a quest without consulting her first, which made her feel disrespected.
One possible reason could be that Percy has unspoken feelings for Annabeth. He might be in love with her but not yet ready to admit it, so he just stares, trying to figure out his emotions.
Maybe she's jealous of Percy's popularity at school. It could be that he always gets more attention, whether it's from teachers or other students.
The plot could be that Percy discovers an old sweatshirt in his attic that turns out to be a magical artifact from one of the gods. Maybe it gives him the power to control water in a new and unique way, and he has to figure out how to use it without getting into trouble with the other gods.
Well, one possible reason could be that Percy made a really stupid decision without consulting Annabeth first. Like, if he went off on a dangerous mission on his own when they were supposed to be a team. Annabeth is a very strategic person and she might feel betrayed or extremely frustrated by such actions.
It could also be due to some sort of magic or mind - control. A powerful enemy might have cast a spell on Annabeth, forcing her to turn against Percy. In this case, it's not really her own will but the result of external manipulation.
Maybe she's scared because the fanfiction portrays Percy in a way that's out of character for him. Like, it might show him being overly aggressive or having strange powers that she doesn't recognize, which makes her feel like she doesn't really know him anymore.
One possible reason could be jealousy. Maybe she sees Percy getting more attention or praise than her in the fanfic world, and that envy turns into betrayal. Another reason might be a misunderstanding. For example, if there's some misinformation spread about Percy that makes Annabeth believe he's done something really bad.
Another possibility is that Percy made a really stupid mistake that endangered himself or others, and Annabeth, being the more level - headed one in some ways, slapped him to snap him out of his foolishness. For instance, if he rushed into a dangerous situation without a proper plan, Annabeth's slap could be a wake - up call.