It has been said that every person born is given a death sentence.
We are immortal.
We all die.
There is no way around the final fate.
Since the dawn of understanding, mankind has grappled with this end fate. The reason why people have finite lives is unclear, considering the world around them has longer lives (and the cosmos, the heavens, infinite).
When intelligence grows, so does one's propensity to find answers to questions that have no proven answers. What is death? What happens at death? Is there a heaven? Is there a hell? Do you remember this life in the next one? Is there a next life?
Literature, religion, introspection, culture, community and ritual all have tried to comfort the unknown reality that death is the life cycle end of our current state of being.
So what if, at the end point of life on Earth, a person must "fight" for his or her next life?
If life on earth was a random joining of an egg and sperm to create a human baby, what if the next evolutionary stage of human consciousness is a connection to a higher plane of existence, a different energy level, a higher consciousness.
The elements of a secondary struggle for life manifested itself on the island. It contained strange electromagnetic properties. It defied the natural laws of physics by being able to move and disappear. It contained ghosts from the past, and strange smoke monsters. People lived, died, born and reborn on the island. And some lost souls were trapped as whispers, unable to move on.
It would seem that the souls of the main characters were trapped in a struggle for their lives, beyond just their human ones. The characters had to create new, lasting bonds with their fellow lost souls in order to fuse a connection of spirit in order to find a place in the after life. Without those strong connections, a soul like Michael, would be trapped in the purgatory sense of nothingness of an island whisper. Those who learned to trust, love and form deep friendships that they could not in their past had the opportunity to live on in the sideways realm, with those deep connections securing their passage into the great white light (symbolic of many near death experiences patients have told their doctors after being revived.)