Why do some people always go back to each other?

So my ex who I remained friends with has got back in contact with a girl he dated first 8 years ago. He filled me in about their entire relationship and it’s weird to me. They went for 3 years without speaking after a huge argument but they both always seem to get back to each other. They’re mutual friends told me that the ex always felt that he was her soulmate but the timing was always off. They’re both in their late 20’s and I feel sad cause I know he is probably going to end up with her.

But also relieved that I didn’t waste my time especially since he always kept tabs on her even though they weren’t speaking. I just wanted to ask about these types of relationships, It seems so much work for a relationship all this going back and forth like it’s unhealthy but also seems romantic. I’ve never had a guy return to me constantly. Has anyone ever experienced that kind of love?
Why do some people always go back to each other?
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