Do you belive in reincarnation? - that the soul lives forever and only uses a body temporarily. So your soul came from another body, and will move to another one after you die.
Well there is this theory. Today, the worlds' estimated population is about 7.5 Billion people. And about a 1000 years ago there were (I don't know the exact number) but MUCH fewer people. Like a couple hundred million. So the theory goes by that there are only a few hundred souls and that we today only have/get a piece of a soul. And thats why maybe we are so shattered. Because we're not even a complete soul.
And to expand on that theory, with my own thoughts, I think thats why we don't just have one ''the one''. I think that there are multiple ''the one''. That there are different ''the one'' for different expiriances. I dont think there is only 1 person you should expiriance those amazing memories with. And maybe thats where the frais ''soulmate'' came from. Like its not just another soul that is similair to your soul, but that you auctully both have a part of the same soul.