Beautiful thoughts by all, put in concrete forms. I would add to the question with the following observations/thoughts: Maybe we are all soulmates of each other. But of course I realize that even though everyone else is our brother or our sister, some people have more affinity to us. Two things come to mind: Perhaps karma in the end is what really explains the affinity. And in modern quantum therms, the concept of resonance. In a resonance situation two objects or systems are “closer” in terms of possible outcomes because the probability function they share is commonly maximized between them.
The bottom line: Soulmates are bound to us because of karmic resonances. But everyone in a sense is our soul mate because everyone has a soul!