I don't think we're going to agree on a definition of "soulmate" in this forum. Personally, I prefer a loose definition based on the idea that two people have a special affinity for each other that apparently goes beyond the usual bonds that are based "only" on sexual attraction, shared interests, and mutual concerns such as family, finances, and/or survival. I prefer such a weak definition because the term "soulmate" is used often enough to warrant some type of recognition of its meaning from people like me, but I don't want to use a definition that gets carried away. I don't believe that two people can literally share two halves of one soul, but I will grant that many people feel like they have an uncommonly special relationship with their partners. Who am I to say they don't?
I have my own ideas about the reasons why some people believe they have found a "soulmate". One reason is because they have an uncanny way of communicating with each other non-verbally. But surely such communication would not be possible if they were not using cues that they had learned while growing up. To me, it's just a happy coincidence that they can "speak the same language" so well. If they had grown up on opposite sides of the world or in eras a few hundred years apart, I doubt that their "souls" could "connect" so easily. By the same token, I think that "soulmates" share values, patterns of thinking, and ideas that are also the products of time, place, and upbringing. If any of us had grown up speaking a different language or if we had not, for example, read the same books we've read, our brains would not be the same as they are now. Even "soulmates" would not be able to have a meeting of the minds. Thus, soulmates are simply two people who coincidentally have thoughts and minds that seem to "fit" together when those two people engage in verbal or non-verbal communication. It must be very nice, but I think they're getting a little carried away if they think it's any more mystical than that.
I also think that identifying another person as your "soulmate" is a projection of your own desires. It's not just an attraction to another person; it's also a desire to have your "whole being" acknowledged and appreciated by that other person. We make ourselves vulnerable when we seek that sort of attention, and the object of our desire takes on an importance that most of the other people in our lives don't have. It's sort of like transferring our need for parental love onto a romantic partner and then expecting that person to meet
all of our emotional needs--not just our need for romantic love, but also our need for basic approval. When our ego gets tied to a person like that, it won't let go, just as our parent-child bonds can never be completely broken. Subsequently, that particular person becomes "unique" and irreplaceable.
Of course, the danger is that the other person might not be everything we expect him or her to be. But once they become a "soulmate", that's it. When it comes to soulmates, a person's
belief in the other person is what gives that person the unique identity befitting a "soulmate". Not only will the heart not let go, but the mind will also not let go of the belief that the other person is a "soulmate". That is a very powerful perception. The idea of a "soulmate" is a very compelling idea, at least to someone who is romantic.
There are times when love grips us unexpectedly and we fall for someone and our emotions are completely out of our own control. And most people have a strong tendency to believe that the people we love are unique. Does that make the other person a soulmate? No, especially not if you do not believe in soulmates. Personally, I think that we are all capable of bonding with more than one person whom we might happen to meet. Instead of falling in love with this person over here, we might have fallen in love with that person over there, and the alternative relationship could be just as pleasing and profound. I think that our biological and psychological dispositions are more accommodating than a lot of people give them credit for. It's lucky for us that we can meet an ideal partner in the same generation and in the same country, or sometimes even in our own hometown, and we're exactly the right age for each other. What are the odds?
If my soulmate is some guy who lived in China two centuries ago, then I guess that means I'll never be happy.