I find that the doctrine of pre-mortal existence, that we were alive and sentient before this mortal earthly life, takes a larger and larger place in my cosmology as its implications sink-in.
When I first encountered it, the doctrine of pre-mortality seemed like just 'kicking the can further back up the road' - but this is not so. It is a profoundly different perspective from (say) that we (our selves) were created from nothing at the time of conception, in embryo or at birth; and profoundly different too from the various theories of cyclical reincarnation and transformation.
Indeed, this doctrine of pre-mortal existence is coming to seem like a key - the necessary first piece of a jigsaw of inference that makes sense of mortal life, and gives meaning to it; and enables a positive attitude towards it.
I have found that rival doctrines cannot explain the necessity and desirability of our mortal life - this actual life we are living here-and-now; so that there is a tendency to regard mortality as a misfortune we would wish to be over as soon as possible, a trial to be endured as a punishment, or simply trivial - swallowed-up by the eternity of post-mortal life or rendered insignificant by the round of reincarnations, or a momentary spark in an infinite universe and duration of nothingness.
But to perceive mortal life as a discrete episode, sandwiched between the two open-ended and unmeasurable eternities of pre- and post-mortal existence - is to change this earthly life from being just an infinitesimally-brief mere-prelude to Heaven or Hell or extinction or a roundabout of similar incarnations - into a infinitely precious - because unique and bounded - era of our existence.
And pre-mortality enables us to consider mortal life as a choice; and for each person to understand himself as a volunteer rather than a conscript in this spiritual and bodily warfare amidst which we find ourselves; because at some level and in some way and with some precision (although not pre-destined) we chose this actual mortal life.
Thus pre-mortality is a way-out from what seems like the logical necessity of this whole business of mortality being an accident, an unfortunate accident - the notion that we were born here-and-now and as who-we-are for no reason at all but simply as the unrepeatable outcome of roll-upon-iterated-roll of genetic dice - and that we are (it seems, in such a mood) nothing-but this combination of randomness, genetics and contingent circumstance...
However, pre-mortal existence locates our essential selves outside of earthly chance and contingency - our essence is prior to randomness, genetics, and environment; our real selves are independent of contingency; and therefore there is meaning and necessity to our basic situation.
Pre-mortal existence is one key part of the doctrinal pre-requisites we require to understand that this actual earthly mortal life of ours is ultimately part of an eternal narrative and striving, with which we personally have already and previously aligned-ourselves.
Note: These thoughts were crystallized by watching a video of the late Elder Neal A Maxwell speaking on the subject of "A wonderful flood of light" - https://www.youtube.com/watch?v=CJwZ6zQhyZ0 - h/t 'Zen' at jrganymede.com. Correction: the poetic quotation Elder Maxwell attributes to CS Lewis was actually by JRR Tolkien (from Mythopoeia) - and was rather addressed-to CS Lewis.