In the beginning, God created spirit creatures.
God gave these spirit creatures the ability to choose their own destiny ? our will. Thus our Souls were born (Special note to JWs: A Soul is not a body)
Selfish actions are the first instinct of any species with a will. Some chose to stay next to God (the Angles in Heaven), many choose the broad path and wound up on Earth. How you ask?
In the early days, our spirits had the ability to push-into and out-of animal matter with free will (much like legion inhabited the swine and the fierce men)? however in our haste, we overlooked the fact our spirits could become enmeshed with animal flesh, and hence we slowly, but surely became trapped in the cycle of birth and rebirth of the animal bodies we used to inhabit for selfish fun. There are vague references to this in Genesis.
One of the main problems is that most people have forgotten their true heritage and place in the Universe as co creators with God.
Have you ever considered what you ?really? look like at the spirit level? Most likely most of us have had hundreds of bodies, and the reason we like our appearance, is that our appearance is the sum-result of hundreds of life experiences? each time, our face getting another smile line, or a frown, or a cancer. Our DNA is not random, it is the sum result of hundreds of our Soul?s experiences. If you shed blood in this life, and you can expect to be reincarnated with a DNA gene that causes your body to shed it?s own blood in the form of leukaemia ? lending teeth to Jesus? argument that those that live by the sword will die by the sword.
Getting back to your original point, Jesus raised this same issue of rebirth with Nicodemus. If you read the passage carefully, you will actually see that Nicodemus reflects Jesus statement back to him in an effort to confirm something his brain does not comprehend. He understood what Jesus said ? he just didn?t ?get it?, and Jesus (for whatever reason) did not feel the need to explain it further. Jesus did this a lot.
I have vague memories of my distance lives. There have been several famous people throughout history that have claimed to remember many of them with clarity. I have no doubt, that when Jesus claimed his existence before Abraham, he was remembering it and spoke with conviction? and again? failed to explain it further to the bewildered crowd.