We take many of the enjoyable things in life for granted, assuming they'll always be there: our children, our food, our gadgets, etc. When we first experience them, we're elated, but as time goes on we habituate ourselves to their presence, causing us to enjoy them less and less.
For example, you really hit it off with someone in your freshman dorm during your first week at college, and for awhile you can't sit still because you just have to hang out with them. You're practically joined at the hip. But as the end of the school year approaches, you notice that while you still eat meals together, goof off in each other's room together, and so on, it's nowhere near as exciting as it was during the first month. You're fine spending time by yourself or with other friends. Suddenly the summer arrives and splits you two apart, and you find yourself wishing you'd created a little more mischief during the second semester, by which time you'd taken the relationship for granted.
The Stoics recommended a technique to prevent this habituation to enjoyable things, or what is now called the hedonic treadmill by psychologists -- negative visualization. You simply think to yourself what could go wrong that would separate you from the enjoyable thing. For instance, you might tell yourself that this could be the last day of your friend's life, or that at any moment they could be called home by their family, and so on. When you discover that these bad things don't unfold, you become grateful for the continued presence of your friend. Doing this regularly (not necessarily every day, but perhaps weekly or monthly) keeps you from growing accustomed to them, so your enjoyment is not diminished by habituation.
In the abstract, this solution makes good sense, but it seems that the power of it derives from being able to truly convince yourself of the non-trivial chance that the bad things may actually happen -- otherwise you will brush those possibilities aside and proceed to take the good thing for granted. Now, the Stoics wrote in a Malthusian economy where growth was never sustained, where class mobility was very limited, and where disease, war, and famine were always around the corner. In that harsh world, it would not have been too difficult to tell yourself that your child might die by the end of the year, that this meal is special because you might be starving next week, or that your newly won status may vaporize when civil war breaks out and you find yourself exiled to the middle of nowhere.
However, convincing yourself of these things in the comfortable modern world we live in now is an impossible sell, not merely a challenging one. There are exceptions, such as high school friends who know they'll be separated upon graduation, entrepreneurs who know their business is likely to fail before long, a captured soldier who knows his survival is sketchy at best, and perhaps a few others. But in general, only the most self-delusional can convince themselves that they might also suffer the same misfortunes that Marcus Aurelius could have expected. For us, the good things appear to be here to stay.
So where can we look for good things that have a built-in expiration date, allowing us to use negative visualization to enjoy them at roughly the same level, without taking them for granted? I think we should look to things that go in and out of fashion, broadly construed. Seneca did not live in a world where there may be great popular music that would last for a stretch but that would be booted out as bad music caught on only for its novelty value. Or where colorful clothing would be the norm for a few years before succumbing to a backlash of black. Or where the "it food" might be steak with bearnaise sauce this decade but rice with soy sauce in the next.
In the modern world, the only good things that may well vanish tomorrow are those subject to the churning of the fashion. The standard silver lining that people see is that, hey, they just might come back into fashion! And don't get me wrong, it is indeed wonderful to hear The Cars, Prince, INXS, and early Madonna over the speakers in supermarkets nowadays. But that doesn't solve the habituation problem, and you have to wait a very long time for the revival -- if it happens at all. I see a better side-effect: knowing they are to be hanged by fashion in a fortnight lets you use negative visualization to enjoy them fully for every day that they do exist. *
Moreover, even when the trend is gone, you'll still have all of the pleasant memories, and even more importantly you won't feel paralyzed later on by regret. You didn't take the good thing for granted, and so there will be no mountain of missed opportunities weighing down on your mind.
I'm not suggesting blindly following fashion, since you may not like what's in vogue. But given all the various sources of pleasure that are subject to fashion, there must be at least a handful at any given time where the hip thing is also the enjoyable thing for you. I feel grateful to live during a time when going dancing at '80s night suddenly became trendy -- you never know, the fad might die out and dance clubs might revive The Spice Girls and Ricky Martin! -- and when, thanks to Starbucks, a coffeehouse craze has become "in" -- and who knows whether that will go the way of the roller rink?
As I said before, there are a few other ways to use negative visualization to your advantage: starting a business, going into a warzone, taking a class, or teaching (your students will only be around for so long). Still, those aren't feasible for most people over the long-term. But fashion's blades will always be cutting short the thread of many enjoyable things. ** And if you just search a little for the trends that appeal to you and remind yourself that they won't be around forever, it isn't so difficult to cherish what you have.
* You might object that you'll know that even after the good thing becomes unfashionable, you can still experience it, and that this awareness will thwart your ability to use negative visualization. In some cases, that just won't be true -- try finding a drive-in movie theater or a nightclub that only plays disco music. Even when the thing is still around, experiencing it just won't feel as good as it did when it was popular and there was a collective euphoria surrounding it. A lot of these things are enjoyable in part for the social connections they create between you and your fellow indulgers. Worse yet is if there's an outright backlash, where all but the strongest will consider their public reputation more than their private enjoyment. Think of wearing colors and patterns during the '90s, or suggesting to your friends that you go dancing when brooding and angst are what's hip. The contemptuous eyes of the mob will spoil your attempts to continue enjoying the good thing once it's passed out of fashion.
** Creative destruction in competitive markets does this too, but it's not possible to pick out which particular things will be driven out in this way -- otherwise you'd have a perfect get-rich-quick scheme. You can only look back in retrospect at a horse-and-buggy and think how nice it was that you got to experience it while it lasted. You had no reason to suspect it would be replaced by a self-propelled hunk of metal. So trying to use negative visualization won't work for good things that will be swept away by even better things through creative destruction.