To dumpers, what caused you to become cold and dismissive of your ex?

Now this doesn’t apply to those escaping abuse or extremely toxic situations, but for those that broke up with someone, what cause you to change your entire perspective about them and treat them so vastly differently to the way you would have before the breakup?

And yes, I’m the dumpee who’s been absolutely shocked by how cold my ex became after the breakup. Especially after I gave them all the love in the world and they dumped me “for my own good”