The heart wants what it wants is often explanation enough,
but for anything it doesn’t want, that phrase is unfairly inadequate.
When falling out of love, the rationalisations we latch onto can only be representative of the underlying friction. The friction itself is usually built from a continuous stream of interactions that only you have been present for—each one providing context for the next.
While you’re feelings have evolved from all of them, many of them you don’t even remember.
You can try and explain to someone why it’s not going to work out—and you should try if they need you to—but whatever you say, how much of the truth can you actually convey?