Nostalgia was originally used to describe a kind of pathological (painful - even potentially fatal) homesickness; but is now much more widely used to mean a yearning for something in the past (time of life, place, person, group) - which yearning may be deep, rich and pleasurable.
Although it is generally regarded as a rather weak and sentimental affliction; I regard nostalgia as, in itself, a Good Thing, especially in potential - because to experience nostalgia is a sign of depth of nature, perhaps evidence of the capability for love; a looking beyond the currently dominating incentives of here-and-now.
But nostalgia is just a beginning; and what matter is what happens next. Like most things, nostalgia can lead to bad outcomes.
For example, it can lead to the attempt to return to childhood, or a futile attempts to rebuild that which has irrevocably gone (and which probably ought not to be rebuilt anyway). This error might be termed "nostalgia as an end in itself".
It may also represent (or lead to) a spiritually dangerous things, which is trying to recover and re-live an earlier stage of oneself; trying to undo and forget (rather than learn from and repent) aspects of one's life.
Or, the opposite; the impossibility of returning to the earlier situation may lead to a kind of despair. The past is seen as entropically-doomed because the world inevitably decays, our memory always degrades, the past is annihilated, death by destruction is the end of all...
This error might be termed nostalgia as a deceptive (because unattainable) vision of the impossible Good.
Either way; nostalgia can lead to a kind of life-sapping attitude - especially in a secular-materialist society such as ours (one which favours and rewards psychopathic callousness); which perhaps accounts for nostalgia's low-status reputation.
If, on the other hand, someone has an assumption of his own life as a stage and phase in the experience of an eternal soul; and has the expectation of resurrected life in Heaven to follow death; then nostalgia can assume a very positive role in life, and beyond.
Nostalgia (potentially) points us in a direction that recognizes the positive value of our own past experiences; and these experiences may include imaginative and empathic experiences; for example the best experiences from reading, arts, television; and the experiences of people with whom we identify (both those people we have known or met, and those about-whom we have learned).
We may realistically expect such nostalgia experienced in this mortal life, to form (in ways not explicitly comprehensible at present) a vital part of our eternal life in Heaven.
