Previous memory has to be wiped out because otherwise you cannot accept the current life. E.g. I had some past life memories and I just could not accept my current family and gender.
A soul learns only from actually experiencing each part of it's godliness. E.g. if it needs to learn to forgive then it must be in a situation where it is wronged. Hence that situation will arise in that soul's life.
Though we come again and again till we reach Nirvana (Become God Again) if we do not clear a lesson in one life...we repeat it in next life.
When something adverse happens to us we must stop and think "what is it that I need to learn from this situation.
The life that we live on Earth is written by us as a Soul on the other side (Pre-birth Soul Contracts) and it is done keeping our best interest in mind. It is a story that is written by US. I call it a contract that we sign before we come on Earth and it is only the decent thing to respect the contract.
Ranjna