One possible reason could be a miscommunication. Maybe Serena did something that Ash misinterpreted. For example, she might have made a decision about their pokemon journey without consulting him, and Ash, being independent, took it the wrong way.
It could also be that there is some backstory in the fanfiction where Serena is associated with a group or an event that Ash has negative feelings about. For instance, if she unknowingly befriended someone who had previously caused trouble for Ash, and he holds it against her.
Perhaps in this fanfiction, Serena has a side to her that is overly competitive towards Ash. This could make Ash feel pressured or annoyed, leading to him developing dislike for her. Like if she always tries to outdo him in pokemon battles or in getting the attention of their friends.
Maybe she has her own vision of their relationship and the fanfiction distorts it in ways she doesn't like.
Maybe Ash is portrayed in a way he doesn't like in Serena fanfiction. For example, the stories might change his character traits or put him in situations he wouldn't be in the original series.
Maybe Ash hates Pokemon fanfiction because it often goes against the official storylines and characterizations he is used to in the real Pokemon world. Fanfiction might distort the relationships between characters or create out - of - character situations for him.
Maybe Ash's character in the original work has a certain fixed image, and some fanfics about Serena deviate from this image he represents, so he hates it.
In many Pokemon fanfictions, Ash might comfort Serena by simply being there for her. For example, if Serena is feeling down about a failed performance in a Pokemon Contest, Ash could offer words of encouragement like 'You'll do better next time, Serena. I've seen how hard you practice.' He might also share his own experiences of facing setbacks in Pokemon battles to make her feel that she's not alone in facing difficulties.
The chemistry between Ash and Serena is really appealing. Ash is always so focused on his Pokemon journey, and Serena brings a different dynamic with her goals related to Pokemon showcases and her feelings for Ash. Fanfiction allows fans to expand on their interactions, like in a story where they might team up against a powerful villain in the Kalos region, which wasn't fully explored in the original series.
A possible fanfiction plot could be that Ash, Serena, and May are invited to a mysterious island full of unknown Pokemon. They have to work together to survive and discover the secrets of the island. Ash's leadership, Serena's ability to communicate with Pokemon, and May's quick decision - making skills would be crucial.
Maybe in the fanfiction, Chloe has some traits or actions that Ash doesn't like. For example, she could be too bossy or interfere with Ash's goals in the story.
Maybe he just doesn't like how some fanfictions misinterpret his experiences in Alola. Fanfictions can often take a lot of creative liberties that might not sit well with the character as he was originally portrayed.
Maybe Ash has a particular view of his own character and the way Cynthia is portrayed in fanfiction doesn't align with his idea. He might feel that the fanfiction distorts their characters or the relationship between them in some way.