If we have lived before, how is it that we do not remember our former lives?
There is great reasoning in why we do not remember past lives. The purpose of reincarnating is to learn from our mistakes and experiences. Imagine in school that yesterday you had taken a test and you saw that you failed when it was reviewed. Now today you have the same test to retake. Well you might have remembered the correct answers but not how to solve the problem. You might have scored well by memorizing the answers but are you any smarter? By failing through multiple lifetimes eventually you will learn that you must make the correct decisions to move forward. In this case it might cause you to study before the test.