My understanding is that we are incarnated into this mortal life after a pre-mortal life as (unembodied) spirits.
This means we are born with dispositions - and sometimes these dispositions may make salvation very unlikely.
(Unlikely but never impossible, else we would instead have become demons, and not been incarnated at all.)
In other words, some people are 'born evil' with dispositions that - unless they change during mortal life - will lead to the rejection of Heaven.
Most obviously evil are those people incapable of love.
Obviously these would reject Heaven and choose hell: reject Heaven because it is a place based-upon an eternal commitment to love; choose hell because without love there is only selfishness and manipulation.
But another group of less-obviously evil are those who are capable of love, but who refuse to repent their sins.
All Men (except Jesus Christ) are sinners, and one who refuses to repent his sins cannot enter Heaven, since that is a place without sin.
Sin is Not to be aligned with God's will; whereas all in Heaven are wholly in accord with divine creative purpose. So all Men (except Jesus Christ) need to be 'purified' of sin before we can enter Heaven - and this requires repentance.
here are various reasons for not repenting sins. Such as a refusal to admit we are or ever have been wrong.
Or the wish to blame other people or circumstances for our sins - which is the wish to be regarded (and regard oneself) as an 'innocent' victim.
Or having made some sin The Most Important Thing in our life; and refusal to give it up...
To refuse to repent is, in effect, the desire to have Heaven (and its inhabitants) re-made around one's sin.
It is implicitly to assert that 'my sin' is more important than Heaven.
But Heaven cannot be remade without ceasing to be Heaven - therefore this refusal to repent is to self-exclude from Heaven.
What happens to one who loves but will not repent? My guess is that such include those people who seek the oblivion of ego-less, passive bliss - oneness with God conceptualized as abstract and impersonal love.
This is perhaps the choice of one who wishes to live eternally 'in' a being-state of pure awareness of love; but who can only achieve this by an annihilation of the self, and choosing the incapacity for action, thought, and agency.
It is a consequence of the 'all you need is love' attitude, which claims that love dissolves sin, and therefore renders repentance unnecessary.
In effect; such a person chooses to live-out mortal life without repenting his sins; and then (rather than give up sin) wishes to cease to be a person and become merely a minimally-aware spirit, unaware of time, in a situation of rest/ peace/ silence and pleasant almost-nothingness - which might conveniently be termed Nirvana.
Therefore some loving people do not want Heaven and prefer Nirvana; and (to recap) I think it will be found that many such people carry with them unrepented sins - which I suggest is the reason behind this preference.
Whether such people will indeed get Nirvana after depends on whether that is truly what they want.
If someone genuinely wants Nirvana instead of Heaven I see no reason why God would not be able to grant this experience (whatever the underlying reality of that experience might actually be).
Yet, as of 2022 - at least in The West - it seems that the desire for Nirvana is often insincere; because in-practice those who profess Nirvana often preach sin - and will publicly defend and deny sin in principle - and thereby endanger the salvation of others.
And this is demonic activity; and demons actually want hell - not Nirvana.