While we're on the topic of theories - here's mine!
I have wondered what is done with all the souls that have been lost - and what becomes of a person's "being" once the person has passed away? Are these souls recycled somehow, or are they completely "dead" as well? I find this to be an interesting topic because, well...we have all seen and heard the little references. For example - a man who drinks directly out of a milk carton and then wipes his face with the back of his hand might have been a caveman in his past life - LOL. (There were a LOT of cavemen!) A woman who knows nothing other than how to be submissive might have been a negro slave in the South in the much earlier days. A diver who spends more time underwater than he does on dry land might have been a shark, or a lovely angelfish.
My point is - how do we act the way we do? Especially when behavior that we exhibit is not necessarily learned? What makes us different? Were we, in fact, different people? I'm sure we were - or if we're "new," perhaps we will be setting a mold for others who will live on with our souls in a future life/installment.