Nostalgia is an emotion much talked about, mostly in maudlin terms and many, indeed most, claim to have experienced it at some point in their lives, but what the vast majority apparently do not realise is that emotions, the strong ones at least, are not inherently maudlin ones. Thus, while it is always thought of as a mild emotion, allowing one to look back at the past through a rosy mist, abstracting away from all the pain and frustration and feeling of the past, those who truly experience it realise, after a point, that it can be a poignant, even a wrenching, emotion, if only for a moment, but what it is not, unless it is not a natural upwelling, is a mild and rosy one.

Nostalgia is more than mere reminiscences, it is a longing, a pining for what one has had and does not anymore, for what was, and cannot now be, an attempt to recapture a moment lost in time. It can be as weak as a recalling of a particular incident, given similar circumstance, or, rarely, a gripping emotion, one that twists ones heart as if it were a wet cloth being cleansed of water prior to its drying, a pang almost physical in its intensity; even though it were to last only a few moments, these moments may feel an eternity. One then realises that emotion can be, almost, a physical thing, something that lives, throbbing, in one's inner recesses, like a beast awaiting its prey in the dark, rushing out when least expected, and shocking one not only by its violence but by its mere presence.

Contrary to what one might assume, it does not require particularly strong circumstances to have these infrequent upwellings, no need for one to be under tremendous physical or mental stress. Rather, it appears to strike one at moments when one is not even feeling lonely, when one is contemplating an event (or not even contemplating anything in particular), when one is merely listening to music, eating, reading of (or just) something. In fact, there is no known event that will indubitably induce these sharp pangs. The trigger might be anything as simple as listening to a song with specific associations, eating a certain type of food, or even in the combination of certain events (such as drinking a cup of Tea, let us say, while listening to a particular song) to a simple smell while passing a bakery/restaurant in a road.

The associations triggered might be as trivial as a cup of Tea shared with a friend, to an event, in the dim past, not even in conscious memory (possibly brooded upon sub-consciously) and it is these simple events, one realises, that one misses.
These trivialities bring out strong emotions, a longing for something that appears, at these moments at least, irretrievably lost, something that one might, at other times, not even feel are important enough to be mourned over, a game of cricket and the subsequent cup of tea at a road-side place, a stimulating conversation with mates, tired after a hot and competitive game, the very comradely silence that prevails sometimes, these appear unimportant but it these unimportant things, one realises too late, are what are torn away from one.
It is not that these feelings are recalled as that they are physically longed for, along with a strong feeling of certainty that these are never to be experienced once more, a spiritual equivalent of the lame man's excruciating itch in a non-existent leg (limb), one that can neither be scratched away nor willed away, at least for the few moments of its existence, and like the imaginary itch, is excrtiatingly pleasurable and has certain elements of pain.
As with any strong emotion, these appear to last only long enough to be delicious, without being long enough to tire one, and one can rarely feel the same longing for the same event with the same intensity,despite best efforts to re-create it, to capture it.
It is only then one realises, with a pang of remorse, that emotions cannot be captured, turned on and off at will, as the Television; that it is its own mistress and, like Beauty (the adjective, not the noun, although there are those who claim there is not much distinction between the both, one abstract, the other its incarnation, both equally capricious!), is capricious and whimsical, and, because of its very nature, cruel, denying us its pleasure when we most crave it.
We, in vain, attempt to recreate it, re-experience it, and, to paraphrase Gray, we attempt the more because we attempt in vain, and we, moreover, know, within, that we attempt in vain, the realisation that it is only the small things in life that are worth enjoying is reinforced.
Thus, the pith seems to be to live life to the fullest, enjoy the moments as they come, instead of attempting to look into the future, or the past; it is in the present that we are destined to live, both spiritually and physically.

The motto of life then might be, as John Donne so poignantly put it more than four centuries ago, "..send not to know for whom the bells toll, It Tolls for Thee"; to capture this moment, to live now, enjoy each emotion anew, rather than worry overmuch about Tomorrow or Yesterday, and to concentrate on the small instances, emotions, since the major ones ,surprisingly enough, live in one's memory but cannot be re-experienced, the thrill of getting the first job (or the first time one achieves one's cherished goal) is reeancted every now and then but never re-experienced, the way a minor, and apparently insignificant emotion, is re-experienced, and with a poignant feeling.