Let me start by posing some questions: Was John the Baptist the same person as Elijah? Jesus said: "For all the Prophets and the Law prophesied until John. And if you are willing to accept it, he is the Elijah who was to come." (Matthew 11:7–14). So John is identified as Elijah who was predicted to come before the Messiah. The coming of Elijah as a the messenger of the Lord was prophesied in Malachi 4:5: "Behold, I will send you Elijah the prophet before the great and awesome day of the Lord comes."
John himself said he was not Elijah when asked by the Pharisees: "And they asked him, 'What then? Are you Elijah?' He said, 'I am not.' 'Are you the Prophet?' And he answered, 'No.'" (John 1:19–23).
So who was John? If he was not the prophet Elijah coming as a messenger, then who was? If John was Elijah why did he deny it? I will state here that I believe that John's spirit was the same spirit (soul) that animated the prophet Elijah on earth. This is confirmed by the message given to John's father Zechariah about John's mission in life: "he will go before him in the spirit and power of Elijah". (Luke 1:17) (as a side note, The Great Gospel of John makes it clear that Zechariah himself was animated by the spirit of Moses, so here we have another example of a soul or spirit returning to another life on earth).
So why did John deny it? I think it was because the personality of John was not the personality of Elijah, even though both were in a deeper sense 'the same person'. If you think about it when a soul is born into the world he or she takes on some attributes of the parents - both in appearance and personality. Also, the circumstances, education and development of a person born of one set of parents at one time and someone else born of other parents at another time are completely different, leading again to personality differences.
So the question arises, if John's personality was quite different to Elijah's in what sense were they the same? I believe the soul, or spirit was the same. So what comprises a unique soul or spirit? Mainly, the heart, the inclinations - these are our most important unique spiritual features, and are perhaps also combined with different and unique measures of patience, wisdom etc, as given by God when the soul was initially created.
Thus I wish to suggest that each of us has the potential to quite radically change many of the deepest aspects of our personality. If you think about this, it is very empowering. It means we are not bound to be who we currently are. We do not need to be forever burdened with the weaknesses or genetic personality attributes we are born with. For example, we need not be quick to anger, if that is something we have inherited; we need not be alcoholics, if that runs genetically in our family. We are not bound to become 'our parents' - this can all be changed by us if we work with God on these changes.
How wonderful is that? We truly can be what we want to be, be who we want to be, at the very core of our being. However, to be meaningful our changes to ourselves should not so much take place in the outer person, but more so within the 'inner' person, then these spiritual qualities will remain with us and would, as with John or Zecharias - shine through, whatever 'outer' personality we may have. Thus, John, on earth, was still John, not Elijah, but the spiritual man within John and Elijah was the same, just the 'outer skin' of the personality had changed. I suggest that as someone living on earth, John/Elijah would act out of the same love, with the same compassion, with the same 'heart' if you like, regardless of his parents and circumstances.
This transformation of the inner person is hinted at in The Great Gospel of John (7.177.17):
"According to My Word, everybody will become a David within himself forever in My Kingdom."
PS. As a note to this, at a deeper level again, we need not have just the one personality within ourselves. Our personality is who we feel we are inside. I can let you know that it is quite possible to be able to identify with a number of different 'inner' personalities - although these will not be necessarily evident to others, they will be evident to you, as you will 'feel' differently. These different ways of feeling - corresponding to different states of your heart - are like adornments to your soul - gems of the spirit if you like. But these gems do not come for free, we must shape them ourselves though our work on our own inner person, and it is a wonderful creational power that God has given us to shape ourselves, not only into one beautiful person, but many (internally).