Jesus answered and said unto him, Verily, verily, I say unto thee, Except a man be born again, he cannot see the kingdom of God. John 3:3 (KJV)
Marvel not that I said unto thee, Ye must be born again. John 3:7 (KJV)
The wind bloweth where it listeth, and thou hearest the sound thereof, but canst not tell whence it cometh, and whither it goeth: so is every one that is born of the Spirit. John 3:8 (KJV)
We all originate form one reincarnational personality. It consists of the sum total of all past experiences, from various perspectives, over eons of existence. Certain vibrations become emotions or attitudes, and group in various ways. Then, they are applied to different roles in different experiences. These differentiated energies continually diversify becoming very complex with no two exactly alike. This is why we have so many distinct personalities and have such diverse life experiences.
It can take many lifetimes to fully develop one specific reincarnational personality. It can also take many lifetimes to undue the bad traits that one personality may have developed or used. There are numerous emotions, attitudes, and points of view combinations that are possible, and each with varying degrees of intensity or emotion.
The development and evolvement of our many different personalities, is to assure the complete understanding of materiality, and its experiences, from every possible point of view. These experiences are not limited only to earth and to humans, but encompasses the ALL of existence. This is also the reason for screening out of past experiences from present awareness. It would interfere with ongoing lessons or further diversification during current incarnations.