There are a lot of different answers that have been proposed for this riddle. My own theory is that our memory works by association. We see or hear something that reminds us of something else in a continuous and unbroken chain of memories. However, this process of memory association can be broken in a number of ways. For example, upon waking from a dream there is no continuity between the waking world and the dream world, and lacking that continuity, we find it hard to recall our dreams. But then we might see something later in the day that reminds us of something in the dream and that one associative link allows the whole dream to be recalled. Traumatic events, physical or emotional, can also cause a break in the chain and cause memory loss. I imagine that death and rebirth are not only traumatic experiences, but also, like waking from a dream, cause the soul to suddenly find itself in an environment very different from the previous environment, and again the continuous chain of associations is broken. Even so, we may see or hear something that stirs a past-life memory, or we may reconnect the links via regression. Another theory holds that as the soul becomes more advanced it is better prepared to deal constructively with past life memories, and so is granted more access to them.