it doesn't make any sense, to me, that one's soul is 'second hand', or even given to you as new at birth, as if it were a teddy bear to be carried through life. We develop, we change, we learn, we err. I understand that, in the belief of many, everything we do is predetermined, but if we're talking in a non-religion specific sense, that a our soul is our personality and the 'essence' of who we are (just like I'm becoming the type of person to use '....' too much) then it just doesn't make sense.
Would and old soul not carry the learnings of it's previous host?
Would a new soul not suggest that the characteristics we have are carried with us throughout our lifespan without changing?
If we look, objectively, at people, their soul is only who they are in the then and there, we all know how something major can shatter a good soul or rebuild a lost one.