If there's anything josei novels have taught me, it's that the men female MCs get together with just don't like them enough.
If they look plain, they look for someone flashier.
If they look flashy, they look for someone plain.
If they behave adventurously, they look for someone stable.
If they behave stable, they look for someone adventurous.
Basically, they just don't love them enough and are instead swept up in temporary passion, as is sometimes normal in real life as well.