It is all hormones, blood sugar levels, and mood swings.
We do whatever our lizard brain decrees and the gray matter works overtime to come up with a rational explanation.
I expect the "soul mate" phenomenon to usually be short-lived. Later selves will deny the experience.
If necessary, the frontal lobes will hallucinate some type of romantic vision to explain our bizarre actions to our rational selves.
Just snuggled with my dog and my OXTR are full of oxytocin. I take it all back. Love is all mystical moonbeams.
"Nothing is true, everything is permitted" - Hassan-i-Sabbah
(or Nietzsche, or William S Burroughs)