Pretty sure it was based on the idea of there being one person that was just perfect for you and that you'd 'complete' each other.
I can't answer any of the other questions, except for the fact that I don't believe there is such a thing as a soulmate. Even if I did now believe that, I haven't found it. Ha!
I don't think there is such a thing as a soulmate. It seems completely ridiculous in my opinion. Out of the 7 billion people in the earth there is 1 soul mate for you? That person would most likely be hidden in some tiny village in Korea that you will never meet. People always seem to find their "soulmates" in the vicinity of their living area which is amusing.
I think it's more of a compatibility issue and a "growing together" issue than anything else.
I have found mine. Actually it was 45 years ago, it just took her a little longer to figure things out. Last year she finally fell in love with me and we no longer have a Platonic relationship. She is a lifelong atheist and it wasn't until I became Born Again (as an atheist that is) that she was ready to accept that idea that we are each others ..... drum roll please.... sans-soul mates. I can't remember which of us thought that one up, but she laughs every time I say it. Yes, I believe in sans-soul mates. I don't believe that we each have only one or that we ever find one of ours.
It's wishful thinking.
I use to embrace this fallacy of a soulmate, but I did away with it even before I became free from all fallacies and myths. When I became free thinking I could not accept the existance a soul, that some one out of all the humans were just my match, as a result I work hard everyday to sustain the relationship that I have with my wife of 9years, somedays I even doubt I made a right decision, ( she has those guts to say that in my face that she thinks she made a mistake about me). Maybe some people do find such a combination, some of us have not ..... even after 9years of trying.
I find the notion that for every person there is exactly ONE perfect match out there--as many maintain--to be ridiculous. There may be several or there might be none. Exactly one for every single person would take planning by a higher power, and must of us here on T|A don't take that hypothesis seriously.
I remember the idealism of earier days, high school, where young girls were optimistic about the 'soul mate'.
I thought I was being kind to them in my attempt to squash such idealism as a young nerd, but they gave every indication of being 'sealed' against any divergence.
In hindsight, I expect that this is in parallel with religious belief, with the same degree of 'sealing'. Such idealism, I expect, could explain the divorce rate, since very few 'mates' will ever be 'ideal, and will by definition have socialization or emotional disabilities that make them 'real persons', and not princes, kings, queens, nuns or saints..;p).
I expect a 'soul mate' is really one who does not amplify, but dampen our disfunctionality, allowing us each to be real, and compassinate.
You know those things about you that no-one else shares and make you different from everyone else? Your soulmate shares them.