Musar על גיטין 39:21

Remember that union is not only physical, i.e. not only the flesh becomes one, but also the souls unite in the Higher Regions. When that occurs one's wife has truly become "בת זוגו." On the rare occasions when such union fails to be achieved the husband terminates the marriage through a decree of divorce, and the wife becomes someone else's marriage partner, for she was obviously not destined to complement her first husband's spiritual needs. It is customary to write the גט, divorce document, in 12 lines in order to make the dissolution of this marriage similar to the way it was contracted, i.e. that union was to be achieved within 12 months. Our sages, in lamenting the sadness of such an occasion, did not refer to the numerically "first" marriage when they said that when one divorces one's "first" wife the altar sheds tears. [This comment applies to someone who has found his בת זוג, something called זווג ראשון, the match approved by heaven, see chapter 45 in Rabbi Moshe Alshich's commentary on Psalms at length on this subject. This editor's translation page 323. Ed.] The marriage in question is not one in which people are involved whose souls are already experiencing the second or third re-incarnation. The terrestrial altar which corresponds to the Celestial Altar (known to Kabbalists as אשת חיל), is truly saddened. The Rekanati comments on the reason why the Torah describes the divorce document as ספר כריתות, literally a "book of separation." The union of the souls of the pair in question took place in Heaven long before their respective bodies joined on earth. Therefore, when separation between the pair occurs a "book of separation" is needed in order to effect this total separation. This so-called "book" is to be understood as an allusion to the emanations "ספירות." What had been joined in Heaven has been torn asunder by a husband who was too insensitive to his good fortune of having found his true בת זוג on earth. The Torah instructs the husband (24,1) that if he divorces his wife וכתב לה ספר כריתות, that the divorce decree in question must be especially written לשמה, "for her name," i.e. for this particular couple. It must contain both the husband's and the wife's name as well as the city in which they reside to show that the separation is only between them and does not affect the דו-פרציפות in the emanations of חכמה and בינה. Thus far the Rekanati.