One possible reason could be a difference in opinion on how to handle a particular threat. Sonic might be more impulsive and want to rush in, while Tails, being more cautious, would prefer to plan first.
At first, it could strain their friendship. There might be a period of coldness between them where they avoid each other.
One possible reason could be differences in their ninja duties. Naruto is often away on dangerous missions, and Hinata might be worried and want him to take fewer risks. This could lead to an argument as Naruto is dedicated to protecting the village at all costs.
Maybe it's about their future goals. Ash is solely focused on becoming a Pokemon Master, while Serena might have other dreams related to Pokemon performances or contests. Their diverging paths could cause friction between them. For example, if Ash's journey constantly interferes with Serena's plans for a contest, it could spark an argument.
One possible reason could be a misunderstanding about Jack's duties as the spirit of winter. Maybe the reader expects him to be around more often, but he has to spread winter across the world. Another reason might be jealousy. If the reader sees Jack being friendly with other beings, they could get jealous and start an argument.
Maybe Tails was tinkering with Sonic's shoes or some other personal item without permission. Sonic values his stuff and his independence. If Tails was messing around with his things in an attempt to 'improve' them but instead caused a problem, Sonic would likely be angry.
One reason could be differences in teaching methods. Hermione is a very studious and by - the - book student, and Snape has his own rather unorthodox and often harsh teaching ways. For example, in the official books, Snape is known for his sarcastic remarks towards students, and Hermione might not tolerate this in a fanfiction scenario. She could argue with him about how his teaching is not conducive to a positive learning environment.
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.
It could also be due to misunderstandings. Fanfiction often plays with the idea of miscommunication between them. Maybe Bella misinterprets Edward's over - protectiveness as a lack of trust in her ability to handle vampire - related situations. Or Edward might misunderstand Bella's need for independence as a sign that she doesn't love him as much as he thought. These misunderstandings can escalate into arguments.
In the fanfiction world, it could also be about their personal feelings. Jacob is still hung up on Bella for a long time, and Leah might see this as a weakness or something that is getting in the way of the pack's progress. Also, Leah has her own emotional baggage, and Jacob's actions might trigger some of her insecurities, thus leading to an argument.
One possible plot could be that Sonic yells at Tails because Tails made a risky decision during a mission that almost put them both in danger. For example, Tails might have tried to take on a powerful enemy alone without waiting for Sonic's backup.