Cause that was never a "true interest" in the first place. It was just based on wrong beliefs, expectation that in reality didn't turn out how they wanted to be

That string of actions resulted in them rewiring their brain, understanding how the interest they thought they posses, is not that at all

It as well can be related to EGO that's a pretty tricky thingy to handle. When EGO tends to chase someone (not real you) it tends to do everything in it's power, until it feels like you posses that person to a certain level

Afterwards, when it realizes how it has some kind of control sooner or later it will start acting totally uninterested, since as I said - the interest was never real in the first place

Rather than that the goals of ego are always something else - control, manipulate, get obeyed etc

So always watch out in reality, where is your interest in someone coming from? Is it based on their looks, or maybe on them not wanting to do anything with you? In both cases it's about your EGO

When something is real you'll know it from the start & be peaceful about it. In most cases it isn't as majority of us (humans :D) have a whole lot of inner work to do on ourselves. Before we can even recognize what "true interest" is!