Why does the soul search constantly for its missing half?

In the natural order of the Universe, everything has a dual nature, positive and negative, masculine and feminine. From the heart of the atom, proton and electron, to the most advanced beings, all that exists is formed of opposite charges in perfectly harmonious balance. 

From this perfect union arose light and life. 

Twin souls, or soul mates, united in love, like the atom, like God, are hermaphroditic in essence because their combined entity encompasses both the male and female. Therefore, in their natural state, two souls, positive and negative energies, male and female in the physical worlds, exist as one essence. But when this dual-energy vortex that is the soul manifests in the physical world, it is temporarily divided into two halves so that it may experience each individual trait and recognise the importance of union. It often happens that the female aspect of the soul is incarnated into a male physical body, and vice versa, which creates pain and confusion for the soul that knows its true gender. Only after a series of incarnations, during which each half of the soul immerses itself in the material world, transcends it and purifies itself, can the two halves meet again and reunite for eternity.

But the material world is powerful, and eventually, each half of the incarnated soul becomes wrapped up in its new body and life and forgets its true spiritual essence. Often this causes it to fail in the assimilation of the experiences of a lifetime.

The one thing a soul never forgets is that it is incomplete, that an intrinsic part of its essence is missing. That is why, as long as it is incarnated in a physical body, the soul searches constantly and unconsciously for its missing half and is often attracted to those beings who remind it, however faintly, of its true soul mate or twin soul. This search is generally in vain, as it is very rare for twin souls to be incarnated in the same place at the same time. When this happens, however, the attraction is immediate and all-encompassing, even though the two soul mates may not know the reason for its intensity. But for the most part, only one soul incarnates at a time, while the other remains in the astral plane.

Migęne Gonzalez-Wippler, What Happens After Death, Scientific & Personal Evidence for Survival, 1997, Llewellyn Publications, Minnesota, U. S. A., Part Two, Kirkudian, 3 Soul Mates, 143-4

Emerging flower of a cockade flower (Gaillardia aristata) | Reinhold Möller | CC BY-SA 4.0