Why did she keep coming back into my life?

While I was at my first job towards the end of high school, I met someone very special who I worked with. We were friends for a while and there was a strong mutual attraction, and I eventually asked her out. But our first couple dates weren't anything special.
We stopped dating because she was also seeing another co-worker too, and because I never asked her out again, she was angry with me until I quit the company a few months later.
She eventually reached out to me 3 months later wanting to try again. We dated for a while and it was fantastic, but she ended things due to religious conflicts.
For two years, we didn't speak but we'd occasionally run into each other. But she eventually messaged me because she felt she had to. After a few weeks of talking, I told her I had been in love with her and she let us date again. For a year we dated, up until she decided to go on a mission and kicked me out of her life again.
Two years after she left, she showed back up in my area, and we had one conversation, but besides that, we haven't spoken.
What I don't understand is she told me she understood why we kept coming back together, but I still don't understand.
It's been a painful and difficult time and I'm lost. I just don't understand why she'd keep coming back.
Why did she keep coming back into my life?
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