It's been said that we are eternal spiritual creatures who, from time to time, choose to have a physical experience.

For me, the definitive work on reincarnation was done by Dr. Brian Wiess, in his book "Many Lives, Many Masters". Dr Wiess is a VERY conservative, conventionally trained and widely published psychologist who avoided the subject of reincarnation like the plague. One day he was using regressive hypnosis on a patient (a young lady in her 20's) in an attempt to find the source of a persistent pain in her abdomen. She had previously been examined by several other medical doctors who could find nothing wrong, and who had told her the pain was psychosomatic. After exploring her childhood, trying to identify some early trauma that could be the cause of the pain without any results, Dr Wiess told her to "go to the source of the pain". Immediately, she was a soldier in the trenches if WWI being bayoneted in the stomach. She was cured of the stomach pains, but continued her sessions with Dr Wiess. Ultimately she was able to recall more than 80 previous lives.

If you have an interest in the subject, I highly recommend the book which is available thru B&N. It's a great read and very informative.