So I was watching Advent Children the other day, and all of a sudden I realized something. That Aeris and Zack are the Lifestream's eqivelent of the unsent. But what I don't understand is that isn't everyone supposed to return/meld back into the planet no matter how hard the try. (ex. Elmyra's husband) or are Cetran race able to maintain there original form/memories within the Lifestream. Can anyone explain the reason for this?