The Veil of Forgetfulness
Why can't we remember our previous lives?
It is only within the Third Density evolutionary process that the Veil of Forgetfulness exists.
If there were no potential for misunderstanding, there would be no experience. It is necessary for the Third Density entity to forget where it comes from so that the mechanism of confusion or free will may operate upon the newly individuated consciousness complex.
So, apparently, the need to make choices without conscious understanding of all the ramifications is a necessary evolutionary step in the growth of an entity. In other words, we need to learn faith. If there were no Veil of Forgetfulness, then one would always understand the ramifications of one's choices, and Life would be entirely predictable. In a predictable system, free will cannot exist.
See Before the Veil.