This suddenly struck me yesterday, when I was considering that time is intrinsic, and creation is therefore irreversible.
We began as eternal spirits; but when we became incarnated (on earth, as mortal Men) this decision meant that we could never return to that pre-mortal state.
What happens 'naturally' (I mean in the absence of following Jesus Christ to resurrection) is that the body dies and the disembodied soul continues in a kind of life that was called Sheol by the Ancient Jews, and Hades by the Ancient Greeks.
Existence continued, but 'the self' was lost; the spirits remained alive, but lost self-consciousness - and therefore the free agency that comes from consciousness. This was seen in qualitative terms of existing as witless, demented ghosts - who were aware on a moment-by-moment basis; but had no memory, no anticipation of future, no sense of eternity.
This is indeed probably what the reality of Nirvana actually is. Since the consequences of incarnation are irreversible; the return to original unconscious passive spiritual life can only be simulated, sibjectively.
I mean, post mortal souls can subjectively experience Nirvana (blissful re-absorption into the abstract divine), they can believe (on a moment by moment basis) that they have re-absorbed into the divine - but the reality will be Sheol.
The experience of Nirvana is that the division between self- and the whole is lost; the individual merges-with, melts-into the divine - where the divine is seen as the totality of everything.
This may be understood to be a state of bliss; and for some people I expect it is - since some people are tormented by their separation from reality, are tormented by consciousness and want it to end.
Whether this unresurrected after-life is regarded as Sheol or Nirvana may depend on the way that the divine is understood: either as an impersonal abstraction (Deism; as with Hinduism and Buddhism) or as a personal God (Theism) - and if personal, further what the nature of the personal God is understood to be.
If the personal God is understood to be a loving Father, and we his children; then the state of souls in Sheol would presumably be pleasant - on a moment-by-moment basis: maybe even blissful. But if God is understood to be otherwise motivated (vengeful, tyrannical etc), then Sheol would be understood as more-or-less miserable.
As for Hell - it is different from Sheol, although Hell is also inhabited by spirits.
My best guess is that Hell is chosen by those souls who wish to retain self-awareness but to work against God's creation. The hellishness of Hell is that of a child of God and a product of creation, eternally in rebellion against these facts.
A parable of Hell (and how it is chosen) is a teenager who hates their loving family; and flees them, and cuts-off all familial relations; in order to live in the worst part of the city as a thief, prostitute, drug addict - and who exploits and is-exploited-by, torments and is-tormented-by, the other denizens.
Such people exist, such things happen; and therefore (by analogy) presumably some have chosen, and will continue to choose, Hell rather than Heaven or Sheol/ Nirvana.
Note added: As is probably obvious - the above is an explanation from my particular Christian perspective. I would not expect those with a different understanding of reality to agree with it!