In order to more fully understand the sanctity of this mitzva, we must first explain that God wished to benefit humanity. He created the world incomplete, to provide people with the opportunity to perfect it and make it a better, happier place, and thereby to merit becoming God’s partner in everything good in the world and experiencing complete happiness in it.
The greatest shortcoming in all of creation is detachment. The one God indeed created everything, but because He concealed His light, all creatures are detached from Him, and consequently from one another. Each creature looks out only for itself, thus leading to all the world’s strife, discord, conflict, and war. It is for this reason that this world is called “the world of detachment” and “the world of deception.” The unity at the root of all goes unacknowledged, thus leading to all the evil in the world. The fundamental principle of the Jewish faith is therefore belief in God’s unity, that there is one God and no other.
This is also why the mitzva of settling in Eretz Yisrael, which links heaven and earth, is so important. The most fundamental detachment is that between heaven and earth, as expressed in the detachment between the spirit and matter, between vision and reality, between Creator and creation. Through the mitzva of settling Eretz Yisrael, it becomes revealed that God reigns over both heaven and earth, and that all earthly matters are connected to holiness. The Sages therefore say: “Anyone who lives in Eretz Yisrael is likened to one who has a God, while anyone who lives outside of Eretz Yisrael is likened to one who has no God…and who worships idols” (Ketubot 110b; see below, 3:15).
The value of unity also underlies the extraordinary significance of the mitzva to “love your fellow as yourself” (Vayikra 19:18), which R. Akiva calls “a major principle of the Torah” (Sifra ad loc.).
We are now in position to understand the great importance of the mitzva of sexual union between husband and wife, as it is the most perfect fulfillment of the mitzva to “love your fellow as yourself,” and it expresses the greatest possible unity, the complete union of two different individuals. The unity is achieved on two levels: it unites husband and wife, and it unites body and soul. Often, the body and the soul are in conflict. The soul longs for good, and the body is drawn to evil; the soul desires eternity, while the body focuses on the fleeting present. The mitzva of ona brings body and soul together, transforming even the evil inclination to good. Through this mitzva, the sublime ideas of faithfulness and unity combine with the greatest physical pleasure. The moral value of absolute devotion combines with the greatest joy (see Zohar I 49a; III 81a-b; Bereishit Rabba 9:7, below, 3:13; Maharal, Gevurot Hashem ch. 43).
