From what I've learned, it has a lot to do with attachment styles.

My ex is avoidant, with some pretty narcissistic traits (love bombing, then refusal to even hug because it's too much).

I was/am anxious, or as the couples counselor told me "clingy."

In our one-on-ones, she summarized up a book we had been assigned (which my ex didn't read lol) that it was a statistic thing. 50% of people are secure style - they meet, and tend to stay together cause it just works. ~25% are anxious, and they do ok together and work fine with secure. ~25% are avoidant, and unfortunately, unless they work towards secure attachments, are pretty much always in and out of relationships. There's a small amount of "disorganized" that has both insecure styles, but they tend towards secure over time.

The result is that the older you get, the dating pool shrinks. There will always been avoidant people available though. Secure style people are great at recognizing avoidant and typically don't put up with their bullshit for long. Anxious attachment though end up with avoidants and it becomes a terrible thing, the anxious will do anything to stay, causing the avoidant to do things out of the relationship more.

If you could guess one common thing amongst avoidants that finally ends the relationship, what would it be? If you said cheating, you'd be completely right. It's really hard to end amicably after that.