I don't believe in the concept of a soulmate; I believe in Karma.
But soulmate, like everything else, is just a word and is therefore only as meaningful (or meaningless) as any of us make it. Personally, I find that term very limiting - I think interrelationships of all beings are far more complex and beautiful than that word encompasses.
I firmly believe that if you move about in the world believing in whatever, then you are very likely to find all sorts of "proof" that whatever it is, is in fact, true.
Confirmation bias, I believe they call it.