One theory of soulmates, presented by Aristophanes in Plato's Symposium, is that humans originally were combined of four arms, four legs, and a single head made of two faces, but Zeus feared their power and split them all in half, condemning them to spending their lives searching for the other half to complete them:
According to theories popularized by Theosophy and in a modified form by Edgar Cayce, God created androgynous souls, equally male and female. The souls split into separate genders later, perhaps because they incurred karma while playing around on the earth, or "separation from God". Over countless reincarnations, each half seeks the other. When all karmic debt is purged, the two will fuse back together and return to the ultimate.