It's a sad fact about life that all things end or expire at some point — some last longer, some are short-lived, but there is no escaping the inevitable. What makes it sad is that everything you want, like, and pursue has this predetermined fate: it will end or expire at some point, and while we don't know when that will be, we know for sure it will happen, sooner or later. (Sometimes we might even be able to influence the timing, but only ever to a certain degree.) But then, of course, it's a welcome fact about life too, that all things end or expire at some point. For all the things we disprefer will meet with the same fate. Again, we might not know when, but we know they will.
Thus if there is an element of surprise here (pleasant or unpleasant), it should only be about the timing, never about the fact itself. Yet when we live our lives, we frequently lose sight of this insight, and then we are caught by surprise not just about when, but that something just ended or expired. We're not used to constant awareness of the finiteness of all things; we're not in the habit of memento mori: contemplating death, the inescapable end and expiry of everything, the limited nature of all things (including ourselves).
There are different varieties of memento mori. One is the fearful sort: fearing death. This fear is foolish, although it is perhaps one of the hardest to control. Then there is romantic fascination, a kind of morbid attraction with decay and decadence, an addiction to what is forever lost and what is impossible to reach. Here the memento mori is rather a kind of escape, a way of not being active and engaged in the present. But I think there is also memento mori as tempering a certain kind of exuberance and blind immersion: not over-enjoying pleasant things because you know they will expire and end; not forgetting in the midst of play and fun that there is a serious job to be done within your life's time, which is finite and of uncertain extent; not gaining a false sense of security because of unthinking trust in the stability of things. This last kind, in all its three forms, is what we're looking for.
When we feel ourselves swept away by intense feelings, this can have a certain timeless quality. (Perhaps that is one of the reasons why people fall into the trap of thinking there is a supreme value, something deep and important about feelings). It lets us forget that we are in the middle of not just one, but many processes: what we were just doing, our day, our many projects that might span stretches of a few days or a few years, the main building blocks of our lives such as relationships or careers, and of course, our life itself as a whole that is made up from such building blocks. It may be a kind of relief to forget all this for a moment.
Memento mori here helps us to see that this is an illusion: neither is any subjective experience, any feeling infinite (it will cease soon enough), nor have all these processes into which we are integrated gone away. They're still there, it's just that we momentarily cannot see them. We've taken a brief vacation from reality. And not just any reality, but the reality of ourselves, the reality of our lives, our character as a person. But ask yourself: if getting swept away by intense feeling appears to be better than being yourself, living your own life to the fullest, then wouldn't it be wiser to change your life rather than run away from it? When you get caught up in excitement (in intemperate delight, anger, or fear), then finding back to yourself in recalling the limits that are imposed on your life is a valuable habit.
Likewise, there is nothing wrong with play, even interspersing it on purpose; but what makes this a good idea from time to time is not that it helps us to escape what is more important, it's because it helps us to enable that which is more important. Living your life is primary; fun and play bring in a relaxing element from time to time; yet what makes it relaxing is not forgetting the rest of your life, but rather the awareness that you are getting on well with that activity. It works only as long as you are comfortable enough with the way you live your life that you don't need to cling to frenetic busyness: as long as you work on that aspect regularly and make good progress, you don't have to fanatically squeeze out every single minute. Because if you do the latter, it will wear you out. The tiring factor is never what you do, but that you do it without change and variation. Play is one aspect of the overarching activity of living your life well, and it needs to be used wisely: balancing work, but not replacing or suppressing it in our thought. Memento mori here protects us from forgetfulness: it reminds us that play has to be a scheduled and limited activity, so that it remains part of your way of living, not an addiction that takes over your life.
And finally, none of the things you get to own will last an infinite period: every kind of food has a use-by date; your favorite clothes will wear thin (or get out of fashion), your car will break down (or become too expensive). Not only things are spurious, but so are any conditions around you: whether the social strata in your society and the education you received underwrite that you're well off or make you struggle every single day; whether you live in a peaceful country or in a country at war; whether even the climate on the continent where you reside will remain the same or change, possibly to the point where it endangers the house you built — nothing lasts forever, not even these large-scale conditions will remain the same. They might require much more time to change, yet change they will.
Nothing is certain just because of the way it is now. Personal connections will either continue or wither away, for people will change, move out of sight, or even die. Wealth and health, power and fame will all come and go and you will quite probably be in a situation to make do without them at some point in your life, even if you enjoyed them for a while. Whatever plans you make for your career or your finances might work out or not, depending on whether the (explicit and implicit) assumptions you made will hold over the long time they have to hold in order to work out.
Even while we routinely trust (as we have to) in the stability of things around us, at least as long as there is no apparent reason to think otherwise, we need to be aware that all such stability is again limited and that there will be a time — maybe close by, maybe still far away — when it will no longer hold. It won't seem sudden and catch us unawares when we're in the habit of memento mori. This is not so much about taking specific preparations: if we wanted to take mitigating measures for everything that simply might happen, we'd become paranoid and invest much more than would be reasonable. Mitigation is not for everything we can think of, but for those cases where we can assess both the probability and the costs, and weigh them against the investment we'd have to make. Memento mori, on the other hand, is for mental preparation: knowing what is important in life and what is not, and being prepared for losses; strengthening your character, your resolve to live your whole life well, including those stretches that lay ahead when some of the certainties you were trusting in are no longer there for your support. The aim is not to manage externals more optimally (which is the job of risk mitigation), but to keep your head and remain yourself when the inevitable happens.
Memento mori, the habit of contemplating the end and expiry of all things, is an antidote that keeps us from over-enjoying pleasant things, helps us not to get the serious job of living our whole lives out of sight, and steers us clear of unthinking trust in the stability of things around us. Escape tendencies, forgetfulness, and false security are all addressed by proper use of memento mori, so it's a technique you certainly want to master. Take care.