It's not that people don't change as a default. Of course change is a constant. And as a point, it's important to keep in mind since ideally, you'll be changing alongside another human, so you need to know that and not come crying about "you're not the same man I married" or whatever.
But they never change for you. And people seem to think that's how it works. At the end of the day, you gotta be able to say "if you change for you, fine, I'm happy for you. But if you go on like this, I got the strength to walk myself out of this mess". Not everyone does.