There is an interesting concept, which may have roots in ancient Asian cultures, that states that individuals in the present are basically recycled souls.
We do not have the individuality that we think we have because we are the continuation of past lives, in different bodies, with different memories, experiences, etc.
It is a form of reincarnation, but different. It states that the soul or spirit is the living being, not the human form that looks us in the mirror each morning. The spirit is a non-physical form that inhabits flesh and bone (or in some cultures all things).
The application of this concept to LOST is interesting because the characters who were brought back together in the end church had very little in common besides the island experience. One would think that in an after life setting, the departed would reunite with the parents, siblings and spouses - - - not a raggy band of misfits.
But step back and view the characters not as human beings but the 10th, 20th or 50th version of a spirit. Spirits whose past lives are repressed until their awakening prior to their next version.
As such, the reunion was not about the human characters but perhaps a long standing family or community of like spirits who have completed their last past human existence and are now ready for the next one.
There is some science to the notion that we are not individuals but a collection of past lives: in our DNA. It contains the genetic material for past generations so in fact we each our living histories of many people's pasts. Whether this past DNA is computer code that has an effect on our mental processes, and how our personality leads our lives, is unknown.
If LOST is viewed not as a human drama but as external spirits riding through a material world it does change the entire outlook of the series.