Why are you guys fighting again?
Misty had a crush on Ash
Serena had a crush on Ash
They unfortunately come from a long running series that replaces the females permanently to keep the produce fresh.
There's no way of knowing if he'll end up with either. Even if the anime drops him, are we sure it will show him grown up, or will it just say "his journey will continue" as he wanders into a sunset with his fuckboy Pikachu?