It could be due to a misunderstanding. Rias might misinterpret Naruto's actions or words, and Naruto, being Naruto, might not be the best at explaining himself right away. This could lead to a rift between them. As a result, Naruto decides to leave rather than cause more trouble. He might think that by leaving, he is protecting Rias or her group from any potential harm that could come his way or that he might accidentally bring upon them.
One possible plotline could be that Naruto has his own mission or purpose that is in conflict with Rias's plans or the world she is a part of. So, he decides to leave to pursue his own goals. For example, he might need to go back to the Hidden Leaf to protect it from a new threat that only he can handle. Another could be that Naruto discovers some dark secret about Rias or her group and can't stay with them any longer. He values his own moral code too much to be associated with something he deems wrong.
Maybe Naruto leaves Rias because he realizes he doesn't fit in her world. Naruto is used to the shinobi world with its own set of rules, values, and power systems. Rias's world, which might be more of a devil - related or some other supernatural world in this fanfic, could be too different for him. He might feel out of place and decide it's best for him to leave and return to what he knows. He could also be homesick for his friends and family in the Hidden Leaf.
Well, it might be that Rias sees Naruto as a rival in terms of power or influence in a new setting. So, she uses her devilish powers to abuse Naruto. In the story, Naruto could start out confused and hurt but then draw on his ninja training and positive relationships back home to slowly turn the tables and make Rias see the error of her ways. Maybe he even teaches her about friendship and loyalty, which are concepts not as prominent in her world.
One possible plotline could be that Naruto leaves the village due to some sort of political unrest. He meets a girl from another land during his travels. They bond over their shared experiences of being outsiders in a way. As they travel together, they face various challenges like bandits and natural disasters. Their relationship deepens through these hardships and they eventually decide to get married. It could also involve Naruto teaching his new partner about the ninja way.
It could also be due to a power struggle. Naruto is extremely powerful and independent. If Rias or other members of the peerage try to control or limit his power in some way, he would not tolerate it. His power is a part of who he is, and he's used to using it freely to protect others. So this could lead to his departure from the peerage.
One possible plotline could be that Naruto discovers some long - lost secret about his mother Kushina and decides to leave the village with her to explore this mystery further. Maybe they are on a quest to find some hidden power or knowledge that is related to Kushina's Uzumaki heritage.
One possible plotline could be that Naruto leaves with Tsunade on a mission to find a rare herb that is said to have the power to heal any wound. Along the way, they face various enemies who also want the herb for their own malicious purposes. Naruto's skills and Tsunade's wisdom would be put to the test as they battle their way through.
One possible plotline could be that Naruto is somehow tricked into leaving with Kisami and Itachi. Maybe they convince him that there is a greater threat that only they can help him face. Along the way, Naruto might start to learn more about their motives and the secrets of their pasts. He could also be in a position where he has to use his own skills and values to influence them.
Maybe they meet when they are very young. Naruto, being an outcast in his village, and Rias, who might be facing some politics in her devil world. They bond over their shared feelings of loneliness and not being fully understood. As they grow up, they help each other out. For example, when Naruto has to face powerful enemies in the Chunin Exams, Rias could use her devil powers secretly to assist him without getting caught.
A plotline might involve Sakura facing a lot of judgment from the village. But she stands strong and raises the child with the help of her friends like Naruto and Ino. They support her emotionally and sometimes even materially. As for Sasuke, he might be on a journey of self - discovery and when he hears about the pregnancy, he has to decide whether his path still lies away from Konoha and his family or if he should change his ways.
One possible reason could be a difference in their goals. Naruto is from the Hidden Leaf Village with a mission to protect it and promote peace there. Rias, on the other hand, has her own responsibilities in the Underworld. Their conflicting duties might lead to Naruto leaving.
In some 'Naruto Rias sick' fanfiction, it could be that Rias is cursed by an enemy. Naruto, being the hero he is, feels responsible for helping her. He uses his knowledge of chakra and seals to try and break the curse. Meanwhile, Rias is weak from the curse and her devil powers are waning. As Naruto delves deeper into finding a solution, he uncovers secrets about both worlds that are related to the curse. This could lead to a series of adventures where they have to overcome various obstacles together, all while Rias is struggling with the effects of the sickness.