Reincarnation is a process of personal and spiritual growth. Under normal conditions, the only reason that we incarnate into a physical form is to learn lessons that can be learned in no other state. With each lifetime that we're reborn, we learn lessons and continue to develop on a soul level.
Our lessons and experiences change who we are. These lessons aren't forgotten from lifetime to lifetime, but become a part of us. When we enter into a new lifetime, we retain the lessons of previous incarnations and continue growing and learning. This process by which our lessons and experiences become an eternal part of our being is known as spiritual alchemy.