"Then said I, Lo, I come: in the volume of the book it is written of me, I delight to do thy will, O my God: yea, thy law is within my heart" (Psalm 40:7-8).
Although these words were written by David, they go far beyond any of his own experiences. By divine inspiration, he was penning the very words of the coming Messiah, whose coming into the world had been already written down in God's eternal Word. These words were later quoted as Messianic by the writer of Hebrews. "Then said I, Lo, I come (in the volume of the book it is written of me,) to do thy will, O God" (Hebrews 10:7). Thus, the basic reason why the infinite God became man at the incarnation was that this was the will of the triune God, planned before the foundation of the world (note I Peter 1:18-19).
But then the Lord Jesus elaborated the human implications of His coming in many other wonderful ways. For example, He said concerning His mission: "The Son of man is come to seek and to save that which was lost" (Luke 19:10). All of us were lost sinners, deserving only to be rejected by God, but He came to save us from our sins.
And that salvation not only included forgiveness of sins, but also the gift of eternal life. "I am the living bread which came down from heaven: if any man eat of this bread, he shall live for ever: and the bread that I will give is my flesh, which I will give for the life of the world" (John 6:51).
Furthermore, this everlasting life begins the moment we "eat of that bread," by faith believing in His name and receiving His gift, and this transaction results in a changed life here on Earth, "I am come," He said, "that they might have life, and that they might have it more abundantly" (John 10:10). "For God sent not His Son into the world to condemn the world; but that the world through Him might be saved" (John 3:17). HMM