Why did God determine that "it is not good for man to be alone"?

The second chapter of Sefer Bereishit tells of the creation of woman, a story which begins with God's pronouncement, "It is not good for man to be alone" (2:18). While instinctively we all readily understand why, indeed, man should not live alone, the question must still be asked, why specifically did God determine that "it is not good for man to be alone"?

The Gemara in Masekhet Yevamot (62b) comments that a man who lives without a wife is lacking "joy, blessing and goodness." The lack of "goodness" is derived from this verse – from the fact that God declared, "It is not good for man to live alone." While it is not entirely clear, at first glance, what the Gemara means by "goodness," and how it relates to "joy" and "blessing," we might understand "goodness" to mean, quite simply, the emotional satisfaction that results from a healthy marriage. Or, "goodness" might refer to a passage one page later in the Gemara (63a), where the Gemara explains the term "eizer ke-negdo" ("a helper alongside him") to mean that a wife assists a man in that she manages the household and cares for his domestic needs. We might similarly explain that having a wife is "good" for man in that she assists him in his day-to-day living. This is indeed the approach taken by Seforno in his commentary to this verse.

Rashi, however, adopts an entirely different interpretation: "So that people do not say that there are two deities: the Almighty is singular in the upper world and has no partner, and this one [man] is singular in the lower world, and has no partner." According to Rashi, the "goodness" which could not be achieved without the creation of man's mate involves the theological repercussions of the existence of only a single human being in the world. Man was created from earth, like the rest of the creatures in the world, but was infused as well with the "image of God," with a divine soul. His job is to unite heaven and earth, to become the meeting point between the mundane world below and the divine, spiritual realm above. If man would not have a mate and reproduce like other creatures, this balance would be disrupted; he would be too close to God, rather than standing at the midway point between God and animal.

While we can understand what Rashi says in explaining this verse, it seems difficult, at first glance, to understand why he resorted to this explanation. It is generally assumed that Rashi prefers the simple, straightforward reading of Biblical text and resorts to Midrashic interpretation only when he felt compelled to do so by some difficulty in the text. Why, then, did Rashi here eschew the Talmud's approach, that a woman is "good" for a man in the simplest and most obvious sense? What compelled Rashi to view the "goodness" mentioned in the verse as a reference to the theological necessity of an equal to man?

Professor Nechama Leibowitz explains that Rashi was troubled by the phraseology of this verse. God did not say, "Lo tov le-adam lihyot levado" - "It is not good for man to be alone," which would imply that God was concerned for Adam's own well being. Instead, He proclaimed, "Lo tov heyot ha-adam levado" - "It is not good for man to be alone," or, perhaps more precisely, "Man's being alone is not good." God here establishes the objective necessity for man's mate, a necessity that relates not to man's own interests, but rather to the needs of the entire universe. The very nature of the world, the purpose for which God created it, necessitated that a balance be maintained between man's worldliness and Godliness, a balance that would be disrupted should there exist only a single human being on earth.