From each life you learn a lesson or many lessons. Every life you live teaches you something. When you are ready you are reborn and experience a new life. Your spirit remembers the lessons you learned but not your past lives. With each passing life your spirit becomes more compassionate and wise until you are as perfect as you can be. By perfect I mean that you will exemplify all the desirable qualities of a person. Because everyone is being reborn everyone else is also learning from their lives and eventually everyone will be as perfect as they can be. In this way Summerland is not the Wiccan equivalent to the Christian Heaven, instead is a place between tomb and womb. Our Heaven will be this world once everyone has learned all they can. Because we all are working towards that, every day this world we are in is more like Heaven.
Unlike the Hindu version of Reincarnation, what you are born as is not a punishment or reward for how you lived your previous life. Every living thing, from a snake to a tree, has a spirit and will be reincarnated. Each life can teach a new lesson. You may be reincarnated as a frog or as a human, or as a dung beetle. What you do in this life does not determine what your next life will be. Any pain, suffering or good fortune you are experiencing is not from a past life - it can either be teaching you something or it may be a result of the Wiccan version of Karma, the rule of three (in Chapter 4).
Source: magic-and-spells.blogspot.com