It could be due to Dumbledore's decisions regarding Harry. Sirius is very protective of Harry. If he thinks Dumbledore is using Harry in some way for the greater good without fully considering Harry's well - being, he would be fed up. For instance, Dumbledore's decisions about Harry's participation in the Triwizard Tournament could be a point of contention for Sirius.
It would probably become more strained. Sirius may start to distance himself from Dumbledore. He might not follow Dumbledore's orders as readily as before.
One reason could be the lack of recognition he gets in the village despite his great contributions. In the fanfiction, Naruto might feel that his efforts to protect the village are taken for granted.
Perhaps Naruto is fed up with Konoha's fanfiction because he wants a new adventure outside of the village. He has spent most of his life in Konoha and might be craving for a change. He could be bored with the same old politics and power struggles within the village. Also, he may have realized that the ideals of Konoha are not as perfect as they seem, and this disillusionment makes him tired of it.
Sometimes, the plotlines in fanfiction can be a bit outlandish. If Ron is constantly getting into trouble in a way that Hermione has to constantly clean up after him, it would be understandable for her to be fed up. This could be different from the more equal partnership they had in the original series.
Well, it might be that the fanfiction focuses too much on Sakura in a way that Naruto feels is unfair to the other characters. Naruto is part of a big world with many important people, and if the fanfic just hones in on Sakura all the time, he could get fed up. Also, if the fanfiction has Sakura constantly putting Naruto down or not appreciating his efforts like in the real story, that could be a reason.
One reason could be that in some fanfictions, the characters of Team 7 are portrayed out of character. Naruto might be fed up if they make Sasuke too brooding in a way that doesn't seem true to the original. Also, if the fanfiction focuses too much on shipping within Team 7, like Sasuke and Sakura, and ignores Naruto's growth and importance, it could annoy him. Another possibility is if the fanfiction makes the team seem too dysfunctional in a way that doesn't match Naruto's view of his comrades.
Some fanfiction writers might make Sirius Black a Death Eater to explore the idea of redemption in a different way. If he starts as a Death Eater, then his journey to the side of good could be a more complex and interesting story. Also, it could be a way to add more mystery to the 'Harry Potter' universe by changing the nature of a familiar character.
Perhaps the fanfiction keeps rehashing the same tired storylines about Konoha. Naruto might be tired of seeing the same things over and over again, like always focusing on the village's dark secrets in a clichéd way instead of exploring new aspects of Konoha and its people.
Well, there are several reasons. Dumbledore is this all - knowing figure in the original series, but in fanfiction, his actions can be seen as manipulative. He might be making decisions for Harry without really considering Harry's feelings or wishes. Also, Dumbledore's grand schemes might not always be in Harry's best interest. Harry might realize this and get fed up. For instance, if Dumbledore is using Harry as a pawn in a bigger battle against Voldemort, Harry would likely be angry when he figures it out. And Dumbledore's failure to prevent certain tragedies, like the death of Sirius, could also be a reason for Harry's disillusionment.
One reason could be a difference in ideology. Harry might have come to believe that Dumbledore's methods, like keeping secrets from him, are not in the best interest of the wizarding world. For example, Dumbledore withheld information about Harry's connection to Voldemort which put Harry in more danger at times. Harry could stand up to him to demand more transparency.