Why do ex's get triggered when the people they left are better off without them?

I can give a few examples off this. My best friend's wife left him a few years ago. Not only is he happy, he is married to someone who is in my opinion superior in every way to his ex, and the reason his ex left him, I'm bored, I want Tyrone, or Jamall's dick, or something equally stupid. So she left him. He married her best friend and had kids with her. His ex is now alone, and has been that way for 5 years now, she's let her self go, she's one of the most depressed people I have seen. And any time we hear about her, she's bitching at some new picture he posted with his kids and wife, how it triggers her. About how she not only lost a husband, but her best friend too.

My second example is more personal. When my wife and I first got together, an ex of mine tried very hard to break us up. She told her that I was abusive, violent, that if we stay together she will die. Anyone who has ever spoken to my wife knows that I'm beyond gentle, I never even raise my voice with her. I've never hurt her. The idea of hurting her physically sickens me. Later when the ex found out that we were getting married, she threatened to kill me, my wife, her family and so on. We obviously reported it. And we hired security for our wedding. But we haven't heard from her since.

I only have experiences with women being like this cause I'm a guy. I'm willing to bet though that men do the same. What I want to know is why.
Why do ex's get triggered when the people they left are better off without them?
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