In fanfic, Harry might be protecting someone or something. If he joins SHIELD, it could put that person or thing in danger. Or perhaps he has a different mission in mind that conflicts with SHIELD's agenda. His magical background, if we're considering a crossover fanfic, might make him see the world in a different way than SHIELD does, leading to his refusal to join.
It could be that Harry has loyalty issues. He might have had bad experiences in the past with groups or organizations, and he doesn't trust SHIELD. For example, if he has been betrayed before, he would be hesitant to join any new group. Also, his own powers and abilities might make him feel that he doesn't need SHIELD and can handle things on his own.
Maybe he has his own principles. In some fanfics, Harry might have a different view of SHIELD's methods or goals. He could be more of a lone wolf type and doesn't want to be part of a large organization like SHIELD.
Maybe he has his own personal reasons. He could be afraid of failure, or perhaps he doesn't like the competitive environment. Another possibility is that he has some moral objections to the nature of the competition in the fanfic.
There could be ethical reasons. Fanfic sometimes involves using characters and settings created by others without proper authorization in a more complex way than just being inspired. Harry might not want to get involved in teaching something that has such potential legal and ethical gray areas.
Maybe he has his own reasons. He might be tired of the responsibilities that come with being a hero. In the fanfic, perhaps he has seen the dark side of heroism, like the constant danger and the loss of personal freedom. He could also be a complex character who doesn't conform to the typical heroic image.
It could be that the fanfic contains offensive content towards them or their fans. Thus, Barry and Deo join to refuse it.
Maybe in the fanfic, Harry has his own ideas and beliefs that conflict with Dumbledore's. He might think Dumbledore's plans are too risky or not in line with his own sense of justice.
Maybe in the fanfic, Harry has some unresolved personal issues. He could be haunted by his past traumas, like the death of his parents and his battles with Voldemort, which make him feel that he's not in the right mental state to get married.
He might think fanfic distorts the original concept.
Maybe he has a very traditional view on literature. He might prefer to stick to the original works and not engage with the fan - created versions. Fanfiction often takes the original story in different directions which he may not like.
He could be seen as an enemy. SHIELD might not take kindly to someone with Harry's potential abilities refusing to join them, and they could view him as a threat. This could lead to them trying to keep an eye on him or even take action against him if they think he's up to no good.
One possible reason could be that fanfiction often takes creative liberties with the original story and characters. Harry Potter might be seen as a very personal creation of J.K. Rowling, and she (or in this case, the fictional Harry Potter) may not want it to be distorted in fan - made stories. Fanfiction can sometimes change the essence of the characters or the plot in ways that go against the original vision.