I'M in a branch of Sainsbury's in central London in the evening rush hour.
Every few minutes the tube station across the road delivers a shoal of commuters - mostly young urban professionals - business types in dark suits and smartly-dressed women with impassive faces.
They snatch baskets at the door and scurry round the aisles scooping up prepared salads, wine and ready-meals for one, bleep them rapidly through the self-service checkouts and leave without meeting the eye or exchanging a word with another human being.
I wonder what they're going home to. In my twenties I thought how wonderful it would be to be independent, answerable to nobody and living alone in an exciting place.
Now, with the benefit of years, I know that anywhere, home or abroad, can be the loneliest place in the world. The truth is, the lonely carry that sense of solitariness within them.
Loneliness is the new pandemic. According to statistics, it touches all ages, from the primary schoolchild excluded from play by the casual cruelty of classmates to the frail elderly who've outlived their contemporaries and spend long tracts of time in what amounts to solitary confinement.
And in between, it afflicts the troubled teen who tries to take his own life, the abandoned parent struggling to raise children alone, the ageing singleton who never found a soulmate; the mature couple whose marriage has become a sterile accommodation with each other, the driven careerist who cannot reconcile to retirement and the bereaved, who strive to come to terms with their loss. All the lonely people.
In the ear-splitting cacophony and ever more hectic pace of modern living, we're becoming estranged from each other. The tight little streets where everybody knew everyone else's business, nobody locked their doors and residents flocked to help in a crisis are dimly remembered history. We're a fragmented society now. In our smart new estates people don't know their neighbours.
Our children no longer play in the streets. Our groceries are ordered online and delivered. Shopping and banking are increasingly impersonal transitions. The local convenience store or newsagent may be the only place we're known by name.
We step from home to car to drive the shortest distance, most vehicles with only a single occupant. We scatter after family meals to our separate rooms to pursue our own interests and our children have more `virtual' friends than real ones.
We have inflicted this isolation upon ourselves in the name or progress.
The naturally gregarious and fulfilled are rarely smitten with this sensation of inner emptiness, or perhaps they hold it at bay with constant busyness.
When my father died I grieved of course, but I was carrying out a career and had a crowded social life.
I gave little thought to my mother's loss of the man she'd loved since their schooldays, or the long hours she endured alone waiting for me to come home from dances and parties.
Selflessly, she never reproached me for my neglect of her. Now I belong to that band of women bereaved of a partner, whose grown-up children are at a distance and their lives irreversibly altered.
In a society built on coupledom the loss of such a one is life (and status) changing. It's the little things that hammer home loss with chest-crushing-force - coming home to an empty house and the absence of another living being to rely on; no-one to share joys and sorrows, triumphs and disasters.
Minor domestic crises take on the proportions of Greek tragedy. Sudden awareness of one's own vulnerability brings the litany of `what-iffery?' What if I cut myself, have a nosebleed, choke or fall? And regrets - of words said, words left unsaid, of time wasted being at odds.
Loneliness is the absence of company, of conversation, of the benison of touch, of the reassurance of love. We're losing the glue that binds society together and the lonely are falling through the cracks.
But for the brave, 'alone' can be the incentive to pack away in memory what can't be mended and forge a new reality.