Reincarnation occurs as karma exists. In absence of doctrine of karma reincarnation, rebirth was just not possible. It is supported by karma and evolution life moves ahead in cosmic life cycle.
Understanding crux of spirituality we shall realize that it is our soul atman, the spirit existing in heart that manifested human form. After manifestation how does the human form move ahead in life! This became possible when we indulged in karma.
Karma is the process, the doctrine through which the spirit, soul atman cleanses itself of dross impurities within. There was no other shortcut method of evolving in life.
When death of human form occurs, the released soul atman immediately manifests a new body to start a new journey of life. The manifestation of a new body by soul atman, the spirit was automatic. None could interfere in this process, not even God Almighty.
After death of human body based on residual balance of karma at the time of death the soul atman manifests the new body. Whether we shall be born in the house of a laborer or a king is solely dependent upon the residual balance of karma of previous life at the time of death.
In simple words the closing balance of earlier life becomes opening balance for next manifestation. Principally it is the child that chooses its parents and not vice versa.
Based on karma of previous life if we are born in the house of a laborer, it is quite possible that we totally changed our destiny by marrying the Princess of the state. On scales of karma, everything is possible. A commoner can always marry the King.