There is no doubt that the "compatible helper" [paraphrasing Gen. 2:18] that G-d has intended for a man may assist him in his task. Yet, there is much more to it than that. A single person is unable to complete the rectification that he is to perform in this world. A soul's heavenly source has male and female halves, which are incarnated into the world as a man and a woman. The incarnation of the two does not necessarily occur simultaneously. As a result, only when the man and woman are righteous do they attain the privilege to meet and wed their real soulmate.
By means of three steps of the marriage ceremony, the three main levels of a man's soul…become bound to those of his wife….
Until marriage, however, a man continues to be a half soul. By means of three steps of the marriage ceremony, the three main levels of a man's soul, the Nefesh, the Ruach and the Neshama, become bound to those of his wife, and they both become one being, with one joint spiritual structure.
Consequently, each spouse may only reach spiritual fulfillment and perfection by means of their union, when their lives are conducted with purity.
Even when the man and woman who get married are not real soulmates - which is often the case in our time - they have to accomplish together a rectification that was assigned to them in Heaven. Sometimes, per divine decree, a person is unable to find a compatible mate. It is nevertheless the unmarried person's duty to keep trying to find a spouse, for one may only reach one's spiritual potential through marriage, and a decree may change at any time that the person achieves whatever rectification is personally required before he can find a mate.
His soulmate is allowed to incarnate with him…in order to help him….
When a man comes into this world without a heavy debt to rectify, he may meet his soulmate and marry her without much effort. The Ari cites the case of a man who sins and has to reincarnate, whereas his soulmate has completed her task in this world and has no further need of incarnation. (Shaar HaGilgulim, 20) In special cases, his soulmate is allowed to incarnate with him, and she will came back to this world with him in order to help him. When the time comes for him to get married, however, he will not find her effortlessly as in the first case, but after an intense search and struggle. Since he reincarnated because of a sin he committed, the Accusers on high speak against him; they want him to be prevented from meeting her, on the grounds that he does not deserve it. So they spread animosity between the couple and they later quarrel. That is why it is written that making couples is as difficult as splitting the Red Sea!
[Translated by Simcha Benyosef]