The reason and purpose of reincarnation is our spiritual education and development. When we were still very young not knowing any better, our parents forced us to enroll in a school. Gradually over time, most of us learned the value and advantages of continuing with a formal education. A few however opted to drop out of school. But further into our future, we saw how those who pursued their studies were able to obtain to better positions and better living conditions while those who did not graduate were consigned to menial jobs and a life of poverty, suffering and want.
It is the same in regard to the life in the spirit, in the World of Reality. Because we seek to advance and evolve, we choose to reincarnate voluntarily. Before we are even born into the world, we confer with our spirit guides, the Lords of Karma, for the purpose of formulating a plan of life -- our fate and destiny. Every situation, condition and experience that comes to us were first pre-approved by us. There before our birth here is where we exercise our free will. Many of us are unable to recall those defining moments in the Spirit World. Our non-remembrance often causes our frustrations and disappointments in life. But all of life is an opportunity for our continued spiritual progress. We have no cause to complain.
For more on this subject, you might want to check out:
http://www.innerquest.org.ph/secretsofthekingdom/iq3261reincarnationkarma.htm