1) Our choices. If a child choses to strengthen the ego, he or she loses their spiritual potential more and more. While genotype (karmic inheritance), phenotype (education) and paratype (experiences) are real influences, what determines our trajectory is how we act in response to these factors.
2) The soul experiences the results of its choices. When returning to a new body, the ego cannot manifest until the personality is formed. Since a personality is always necessary, the ego must inevitably return and reincorporate. While the soul experiences some reprieve during the first years of infancy, desire must show itself once again in accordance with karma.
For thirty years I sought God. But when I looked carefully I found that in reality God was the seeker and I the sought. -Bayazid al-Bastami