Man is born in our world through the joining of a physical body with its spirit, which can at one moment be at a certain level in the spiritual worlds. Why then is there a need to start from the beginning in the physical world?
All the births in our world signify a correction of the corporeal level of desires. Even if in the past a person studied Kabbalah and reached a certain level, but not a final one, he must be born as a baby, suffer, learn, arrive at Kabbalah from the beginning, but very quickly return to the level of his previous life and then move on. There is never a repetition, only new corrections, additions to the previous life.
Courtesy of http://www.youtube.com/watch?v=-dIf2SFLwdw