A while back, my sister attended a reunion of her eighth-grade class and returned with snapshots. These women had not seen one another for over half a century. As she’s four years my senior, of course I don’t have her memories. The pictures of sedate women in late middle-age smiling with wine glasses should have meant nothing to me. However, when my sister pointed to one or another and said their names, I was overcome by a rush of irrational sentiment. Carole-Ann, Kathy, Marsha—I remembered them and, for no good reason, felt intensely nostalgic. The names conjured up my old neighborhood with its railroad bridge, the three-storied public school with its granite floors and wide staircases, the tender and ferocious women in tailored suits or sweater-sets who taught there, the slate on top of the schoolyard wall, the hopscotch courts, dodge-ball circles. I remembered what all these former girls looked like when they were eleven years old, even where they’d lived. What moved me wasn’t anything exceptional that happened back then but the ordinary life of my sister’s and my shared childhood.

The ordinary is only ordinary because it hasn’t yet been taken away. So long as the ordinary remains commonplace you don’t notice it. Nostalgia is a sort of noticing after-the-fact, a love for the vanished ordinary. One might recall with either pleasure or pain an adventure, a hilarious moment, an illness, spectacular failure, public humiliation, a good party, or an exceptional meal. But none of these evokes nostalgia. Such recollections are short-lived, while nostalgia is less focused, about something continuous rather than unique. The extraordinary is singular; recalling things like your first time on a two-wheeler, hearing a Beatles song, or reading Crime and Punishment stimulates excitement, the echo of old thrills. But nostalgia is what you feel about steaming wet pavement after an August downpour, the streets you walked to and from school, the bouquet of a bowling alley or chill of a skating rink on adolescent Saturday nights. Any memory is liable to stir up nostalgia but it’s more likely to if it’s of something that was small and happened more than once, like seeing a person you loved before he or she saw you. Nostalgia stretches the ordinary so that what might have occurred only a few times seems to have happened all the time: your father’s awful puns or your mother’s affectionate chiding, your newborn’s crying in the middle of the night then—it seems just days later—reaching for your hand as you’re about to cross a street.

It’s to be expected that nostalgia will fixate on holidays that come back every year, whose rituals merge into collective plurals, rather than one-offs like your daughter’s first piano recital or her senior prom. Once they are irretrievably past, it’s the commonplace motions and conditions of parental life—anxiety, discipline, entertaining, even disputes—that fuel nostalgia, which is easily ignited. An example: my daughter recently mentioned that she’s training the messier of her two sons to clean the apartment, Oscar to his brother’s Felix. “Do you remember our Saturdays?” I asked, suddenly swamped by rich detail. I’d drop her off for her piano lesson, speed to the market, run through the aisles grabbing a week’s supplies, then hurry back to pick her up before the fifty-minute lesson ended. In my nostalgia, I could even hear her and Ms. Ross playing a four-handed arrangement of a particular Brahms’ waltz as I waited at the door. They didn’t practice that waltz every week, of course; but that’s what nostalgia does, turns what was once an ordinary moment into something that feels timeless. On Saturdays, we’d unload the groceries, refill the bare cupboard and empty refrigerator, rejoicing in the abundance. Then we cleaned, each with our his and her chores. Since we agreed it was the worst job, we alternated doing the bathroom. All this was hard at the time—single parenting and the pressure of an exacting job, the anxiety that went with both, and the near certainty that every failure of my fathering would come out on some shrink’s couch in twenty years. Recalling those Saturdays made my heart ache a little, made me smile too. Perhaps it was the same with my daughter. Nostalgia, like misery, loves company.

Nostalgia has more triggers than French biscuits dipped in tea. Because the ordinary is not always good, nostalgia is not exclusively about pleasant memories. I feel nostalgia when I think of the clean smell of the bathroom when I got back from weeks of camping but also of the stifling heat and humidity of a sleepless summer night in Philadelphia. It’s a peculiarity of nostalgia to find pleasure in recalling something miserable. You might experience a wave of it if you run into somebody from your fifth-grade class you thoroughly disliked, even if he had been a nemesis, a bully. What matters is that you watched the same clock on the same wall, took the same math tests, stared longingly out the same high windows. I don’t think this sentimentality is due to any longing to be a fifth-grader again, or a new parent either. What’s behind these pulses of emotion is an affection for how we once were, a fondness for the objects, people, and noises in the midst of which we once lived, also indelible scents like the burning of leaves every autumn. Or maybe it’s more ego-centered than that, a kind of indulgent fondness for our earlier selves.

Nostalgia is certainly part of the human condition, but it may not be unique to our species. Animals live in the present and we admire them for it, find it innocent and charming. We like it when they remember us, when your loyal terrier leaps with delight as you come through the door, when your cat curls up on your lap and purrs. But is that gladness and comfort in your presence owing to nostalgia, the satisfaction of longing for your presence, or just an attachment to the giver of food, the taker on walks and stroker of fur, the instinctive submissiveness of a beta for an alpha? Is it love? If our pets do feel a kind of nostalgia, then it must be a longing for what is ordinary to them, for the familiar, and that’s why they’re so delighted to come home after a week away. There’s the good old rug, the bowl, the door, the sofa that smells just as it should.

Anyone might think that nostalgia is a yearning to revive the past. If so, it is a forlorn fantasy, the pining that leads Gatsby to deny Nick’s objection that you can’t relive the past and insist “Why of course you can” or the doomed determination of the poet in Kierkegaard’s Repetition to do the same thing. But these young men are not so much trying to relive the past as looking for another shot at a woman. It isn’t the past they want to seize but a lost future. They try to compel the past to come around again like the golden ring on a carousel so that this time they can seize it. What they regret and yearn for isn’t the ordinary but the magnificent promise of the green light.

In this season of pestilence, as space closes in and time dissolves, I expect many are nostalgic not just for days without sirens, but once-hated office routines, crowded subways, raucous school buses, and think wistfully of classroom ennui, gridlocked traffic, even the soot missing from the air of still and silent cities.

Robert Wexelblatt is a professor of humanities at Boston University’s College of General Studies. He has published seven fiction collections; two books of essays; two short novels; two books of poems; stories, essays, and poems in a variety of journals, and a novel awarded the Indie Book Awards first prize for fiction.