Perhaps they come from a background where they feel they haven't earned the right to hold such a significant symbol. Maybe they were an outcast or had a history of being on the wrong side before joining the Avengers, and now, even though they are part of the team, they still struggle with self - worth in relation to the shield.
It could be that the character doesn't believe they have the moral integrity that the shield symbolizes. They might have seen themselves as too flawed, like having a temper that could lead to bad decisions, and thus think they don't deserve to carry the shield.
Maybe the character has made some mistakes in the past and feels unworthy of the shield, which represents honor and responsibility in the Avengers. For example, if they had once let their comrades down during a crucial mission.
Maybe it's a new character created in the fanfiction. A character who was brought into the Avengers under unusual circumstances, like being from an enemy faction initially but then defecting. They might feel that their past actions, despite their change of heart, mean they don't deserve the shield.
His past experiences of being an orphan and often being shunned or bullied as a child might resurface in the fanfiction. These memories can make him feel alone even when he is surrounded by people. He might feel that no one truly understands what he has been through.
Miscommunication. The Avengers could receive wrong information about Steve's actions or intentions, leading them to believe he's a danger and thus hurt him.
One reason could be a false accusation. If Spiderman is wrongly accused of a crime by the Avengers or someone within the Avengers' circle, he might feel betrayed and lash out at them. He's just a young hero, and such a false accusation could really make him angry.
He might dislike it because it can create false expectations. Fans read fanfiction and might start expecting the real Avengers to act in a certain way that is not in line with their true nature.
Power struggles can be a reason. Spider Man, being a solo hero with his own set of powers and moral code, might not agree with Shield's more bureaucratic and sometimes secretive ways of handling things, so they clash. For example, Shield might want to control Spider Man's actions, which he resists. This can lead to a fight. Also, there could be a villain manipulating events behind the scenes to make it seem like Spider Man is against Shield or vice versa.
In some fanfictions, Peter could have a secret that he's keeping from the Avengers. When it's eventually revealed, they might feel betrayed and turn against him. It could be something related to his powers, or his past associations that the Avengers find unacceptable, thus resulting in their hatred.
One reason could be that the fanfic writer wants to deviate from the original narrative where Naruto gradually gains acceptance. They might want to put him in a situation where he has to struggle more with being unwanted to make the story more interesting. For instance, they could write a scenario where new characters are introduced and they shun Naruto for some reason.
One reason could be his status as a jinchuriki. People might be afraid of him because of the Nine - Tails inside him, so he feels unloved. Another reason is that he was an orphan, and without his parents around, he may have felt like he didn't have that family love that others had.
Differences in ideology can also play a part. Tony might have some views on how to handle threats or use technology that the other Avengers don't agree with. Maybe he's more willing to take risks with untested tech that could endanger the team or civilians, which would lead to resentment.