Maybe the loss of a very dear Pokémon friend. If something bad happened to a Pokémon he was extremely close to, like it getting seriously injured or lost forever, it could be so devastating that he loses the will to continue his journey and thus gives up.
One possible reason could be a lack of time. Maybe Ash has become too busy with other aspects of life, like work or studies, and can't find the time to write fanfiction anymore.
It could be that in this fanfiction, Ash had some personal issues come up. For example, he might have received some bad news from his family back home that made him lose focus on training and eventually give it up. Another reason could be that he found a new passion or interest that diverted his attention away from training.
It might be that Ash has other priorities now. For example, real - life commitments such as work or study have taken up too much time, leaving no room for writing fanfic.
It could also be due to some external influence. Perhaps an evil organization has manipulated Ash in some way, forcing him to act against Alola. They might have used mind control or blackmailed him with something he holds dear, like the safety of his Pokémon.
His Pokémon would be very confused and sad. They are used to following Ash on his adventures and having him as their trainer. If he gives up, they might lose their purpose and direction.
It could also be due to some external factors. Maybe there are threats from dark forces that he doesn't want to involve Lily in. If he is getting deeper into the dark arts or some dangerous situations, he might choose to give up on her to protect her from the potential harm that could come her way because of his associations.
One possible reason could be that he experiences a great loss, like the death of a dear friend. This might shatter his confidence and make him lose his passion for being a ninja.
One possible reason could be that in the fanfiction, Ash might feel like Charizard has become too powerful and uncontrollable. Another reason could be that Ash has some new Pokémon that he wants to focus on more, and he thinks Charizard is holding him back in terms of training and strategy.
Maybe Naruto gets tired of always chasing after Sasuke. He has his own life and dreams too. He might think that he has sacrificed enough of his time and energy on Sasuke, and it's time to focus on himself and the village. He could start to believe that Sasuke has made his own choices and he can't keep forcing Sasuke to come back.
It could also be due to a misunderstanding. Some false information might have spread in the fanfic's world, making Naruto believe that everyone has turned their backs on him. Like, some bad guys framed his friends, and Naruto, being unable to see through the lies at first, gives up on everyone in despair.