One reason could be that Loki sees something special in Peter. Maybe Peter has a unique power or a pure heart that Loki is drawn to, and he wants to keep that all to himself.
Perhaps Loki is possessive of Peter because he views Peter as a symbol of something he desires, like innocence or heroism. Loki, being a complex character, might want to possess Peter as a way to be closer to those qualities. He may think that by having Peter near him and being possessive, he can somehow absorb or be part of those traits that Peter embodies.
One reason could be that she has deep - seated feelings for him. Since Naruto has grown into a strong and kind - hearted ninja, Ino might have fallen for him hard and her possessiveness is a result of her love. She doesn't want to lose him to someone else.
In fanfiction, Tony being possessive of Peter can be seen as a form of over - protection. Tony, with his genius intellect and resources, might feel the need to safeguard Peter from the dangers of the world. It could also stem from a sense of mentorship and seeing Peter as a younger version of himself that he wants to guide and keep close.
In some fanfictions, Loki's possessiveness of Peter can be seen in the way he protects him. He'll go to great lengths to keep Peter out of harm's way, but not just because it's the right thing to do. It's also because he sees Peter as his own. He might be overbearing at times, not allowing Peter to take risks on his own, which shows his possessive nature.
Love could be the main reason. Since he loves Annabeth deeply, he doesn't want to lose her, so he becomes possessive.
In fanfiction, Tom Riddle may be possessive of Hermione because she represents something he can't fully understand or control. Her moral compass is so different from his. He may think that by possessing her, he can gain some of that goodness or use it to his advantage. Also, Hermione's popularity and influence among her peers might make Riddle want to have her as a trophy of sorts, to show his superiority over others in the wizarding world.
Maybe it's because Hermione has unique qualities. Her intelligence and strength might attract James, and his possessiveness could stem from a fear of losing someone so special.
One reason could be that Fenrir sees Hermione as a unique and powerful individual. Her intelligence and magic might attract him in a way that makes him want to possess her, as if she's a prized possession that no one else can have.
The danger that comes with being Spider - Man also makes Peter a loner in fanfiction. He doesn't want to put anyone he cares about in harm's way. So he keeps his distance from friends and potential love interests. He might not join group activities or go on dates to protect others, which makes him seem like a loner.
In the fanfiction, it could be that Jason has been through a lot with Percy. They might have shared many adventures and experiences that made Jason feel like he has a sort of claim over Percy. It could also be related to the world - building in the fanfiction. For example, if there are certain power dynamics or alliances that make Jason protective and possessive of Percy. Another aspect could be that the author wants to explore a complex relationship dynamic between the two characters, where Jason's possessiveness adds an interesting layer to the story.
Maybe he finds Stiles' character in the original work very interesting and complex, and the fanfiction allows him to explore more about Stiles in different scenarios.