One possible reason could be differences in their approach to fighting Voldemort. Sirius might be more reckless and Harry, having faced more direct threats, could be more cautious. Another reason could be Sirius' over - protectiveness of Harry. He might try to stop Harry from taking certain risks, which Harry sees as necessary for his growth and the defeat of the Dark Lord.
One possible reason could be jealousy. For example, if Harry gets more attention in the wizarding world for his achievements, Ron might feel a bit left out and jealous, leading to an argument. Another reason could be differences in their approach to handling a dangerous situation. Ron might be more cautious while Harry could be more impulsive, and this clash of styles can cause them to argue.
One reason could be Harry's impulsiveness. Harry often jumps into dangerous situations without thinking, and Sirius, being his godfather and having a protective nature, would shout at him to make him realize the risks. For instance, if Harry decides to go after a Death Eater alone.
One possible reason could be jealousy. Maybe Ginny sees Harry getting too friendly with another female character in the fanfiction world. Another reason might be differences in their future plans. For example, Harry may be too focused on his Auror duties and Ginny wants more family time.
One possible reason could be miscommunication. Maybe Harry does something that Sirius misinterprets as a betrayal or a stupid mistake, leading to Sirius lashing out.
Perhaps they argue over a girl. They are both heroes and might have feelings for the same person. Jealousy and competition could spark an argument between them. Also, their loyalty to different camps, Camp Half - Blood and Camp Jupiter, could be a source of conflict. They might have different ideas on how to handle the relationship between the two camps, which would lead to arguments.
It could be due to external pressures in the fanfiction world. For instance, the media or fans in the story might be putting pressure on their relationship. If the reader feels that Suga is not handling this pressure well or is making decisions that are affecting their relationship negatively because of it, an argument can break out. Also, if there are secrets between them in the fanfiction, like Suga hiding something about his career plans that will impact the reader, when it comes out, it can lead to a big argument.
One reason could be for shock value. Writers might want to create something that stands out and grabs the attention of readers in a very different way. Another reason could be to explore a unique power dynamic between the two characters. Maybe the writer wants to show a different side of their relationship, like Sirius being in a more authoritative position over Harry in an extreme way.
One reason could be some sort of ancient magical law or prophecy. Maybe there's a hidden clause that says for the greater good of the wizarding world, they need to be married. It could be related to some power or protection that their union might bring.
In some fanfictions, it could be due to dark magic influence. A spell or a dark force might have corrupted Sirius's mind, making him turn against Harry.
One reason could be the strong bond between them in the original story. Fans might have felt that it could be developed into a father - son relationship in fanfic.