Great question that many have asked before. The most common answer involves several facets of the same idea:
- Adam and eve were not to be viewed as separate creatures but mysteriously united in both function and origin
- Adam was not a complete creature in himself but need woman to complete him
This appears to be confirmed by God's statement after the creation of woman from man. Note the sequence as recorded in Gen 2:21-24
So the LORD God caused the man to fall into a deep sleep, and while he slept, He took one of the man’s ribs and closed up the area with flesh. And from the rib that the LORD God had taken from the man, He made a woman and brought her to him.
And the man said: “This is now bone of my bones and flesh of my flesh; she shall be called ‘woman,’ for out of man she was taken.”
For this reason a man will leave his father and mother and be united to his wife, and they will become one flesh.
Thus, the sequence is:
- a part was taken from Adam's side
- woman was created to be part of Adam
- the two shall be one flesh
That is, neither man nor women can function without the other.
The Pulpit commentary captures this idea well:
The peculiar phraseology employed to describe the formation of Adam s partner has been understood as referring to the physical configuration of woman s body, which is broadest towards the middle (Lyra); to the incompleteness of Adam's being, which was like an unfinished building until Eve was formed (Calvin); to the part of the female in building up the family (Delitzsch, Macdonald), to the building up of the Church, of which she was designed to be a type (Bonar); - yet it may be doubted if there is not as much truth in the remark that "by the many words used in the generation of mankind, as creating (Genesis 1:27), making (Genesis 1:26), forming and inspiring (Genesis 2:7), and now building, Moses would set forth this wondrous workmanship for which the Psalmist so laudeth God," Psalm 139:14 (Ainsworth). And brought her unto the man. I.e. led, conducted, and presented her to Adam. "The word implies the solemn bestowment of her in the bonds of the marriage covenant, which is hence called the covenant of God (Proverbs 2:17); implying that he is the Author of this sacred institution" (Bush). On awaking from his slumber Adam at once recognized the Divine intention, and joyfully welcomed his bride. Genesis 2:22