Perhaps in the fanfiction, Sirius is mad at Harry because Harry has befriended someone Sirius doesn't trust. Sirius, being protective of Harry, might see this new friendship as a threat. He could be worried that this person will lead Harry astray or put him in danger, and his anger towards Harry stems from his concern for Harry's well - being.
If Harry were to break a promise he made to Sirius. Promises are important, and if Harry fails to keep his word, Sirius might get angry. For instance, if Harry promised to stay out of a certain place or not to use a particular magic, and then did it anyway.
Sirius Black may hate Harry Potter fanfiction for several reasons. For one, he values his own identity and the events that actually happened in his life. Fanfiction can create alternate versions of him that he may find offensive or inaccurate. He also had a strong sense of loyalty and friendship, especially towards Harry. If fanfiction portrays these relationships in a wrong or overly dramatic way, it could be off - putting to him. Moreover, he was part of a real struggle against the dark forces in the wizarding world. Fanfiction might simplify or romanticize this struggle in a way that he doesn't respect, as he lived through the real and often terrifying experiences of that fight.
It's popular because their relationship in the original books was full of potential that wasn't fully explored. Fans love to see more of their interactions and the growth of their bond.
In some fanfictions, Ginny might be mad at Harry because of a misunderstanding. For example, she could misinterpret his actions towards another girl as being unfaithful. Maybe he spends a lot of time with Hermione on a project and Ginny thinks there's something more between them.
Fanfiction about these three can take many forms. For example, there could be time - travel fics where Harry goes back in time and meets Regulus and Sirius when they were younger. This could lead to all sorts of interesting plot twists, like Harry trying to prevent Regulus from joining the Death Eaters in the first place. Or it could be a fic where Sirius survives and he and Harry explore the secrets of the Black family together, including things that Regulus left behind. There are also fics that focus on the emotional bonds between the characters, showing how the different personalities of Regulus, Sirius, and Harry interact and influence each other.
It could develop in a complex way. The spanking incident might initially create tension between them. Harry could be angry or resentful towards Sirius at first.
Another aspect of this fanfiction could be that Harry uses his Parseltongue ability to communicate with the snakes in Azkaban (if there are any). The snakes might have some information about Sirius since they can sense the emotions and secrets of the prisoners. Through this unique method, Harry gets closer to proving Sirius's innocence.
In some fanfiction, Harry might be written to hate Sirius due to misunderstandings. For example, if a writer wants to create drama, they could make Sirius do something that Harry misinterprets. Maybe Sirius makes a decision about Harry's future without consulting him first, leading Harry to feel angry and resentful.
Well, a popular one is 'Sirius and Harry: A Different Kind of Family'. In this fanfiction, Sirius actually gets a chance to be a proper father figure to Harry. It shows their relationship evolving in a very heartwarming way. There's also 'The Sirius - Harry Connection'. This one plays around with the idea of a magical bond between them that goes deeper than what was shown in the books. And 'Harry's Refuge with Sirius' where Sirius provides a safe haven for Harry during tough times at Hogwarts.
Maybe he loves the idea of the Firebolt in the fanfiction. It could be that the stories expand on his experiences with the Firebolt in a really exciting way.