The delight has been irresistible. But how will this Olympic euphoria seem to us in 15 years' time?

Collective passion can spread like a contagion. It is impossible to resist

Doreen Lawrence holding the Olympics flag at the Opening Ceremony (Getty Images)

On a sunny evening in early September, 15 years ago, I stood among the trees near Kensington Palace and watched the banks of memorial flowers rising by its gates. Families and groups of friends sat and picnicked on the grass. Some in the crowd had brought hand-made memorials to Diana, Princess of Wales – killed in a Paris underpass a couple of days beforehand – but many had not. I found that, beyond a hard core of mourners, many had come precisely because they knew that so many others would come. They wanted to share a moment and seal a memory.

Were they rubberneckers, opportunists, voyeurs who wished to piggyback on genuine grief? Not at all. These dilettantes of public emotion, probably far more numerous than the Diana devotees, had created the meaning of an event that they came to observe.

Collective will, collective passion, can spread like a contagion. How far can we separate the true believers in the Olympic and Paralympic flames from the millions who felt in some way warmed by their heat? We need not bother. The delight, the enjoyment, the solidarity, has caught on like brushfire. It crackles as I write as, along the Strand, thick crowds greet athletes, volunteers and staff as the floats roll by in the closing parade. It is impossible to resist.

But other kinds of pooled passion can catch fire as well. In a smaller, malevolent way, the London riots of year ago showed us the downside of such mass infection. Beyond the minority of gang members, of dedicated wreckers and looters, a wider penumbra of curious "tourists" joined the mayhem. Ordinary kids committed acts of vandalism or even violence that they struggled to explain. How many of Diana's elegists could, a decade on, have told you exactly why they cared so much? And how will this summer's Olympic euphoria seem to us when we look back on it in 15 years' time?

Perhaps we tend to read such hinge events back-to-front. Diana's death displayed on a wide, lurid screen the ways in which British society had already changed – more expressive, less deferential, adept at marrying a generous humane idealism with rampant celebrity-worship – rather than how it might in the future change. If so, then maybe we should look back at the Olympic and Paralympic summer not as a time when Britain altered, but when we noticed that it had.

Much of this recognition has to do with a culture of inclusion, of active acceptance rather than passive tolerance. A Mo Farah or an Ellie Simmonds, a Jessica Ennis or a Sarah Storey, no longer has to knock on closed doors in the hope of finding a marginal role within a pre-existing narrative of community. They make and tell that story: not as bit-players, but as leading parts. Danny Boyle's opening ceremony struck this note with the big, warm hug of its historical surrealism. Then, as life – or sport – ratified art, events from pool to track to velodrome confirmed it time and time again. This was for everyone.

Before flying home, I watched Boyle's impishly intelligent extravaganza in a hotel lobby in a genteel spa town in central Italy. Many people will have memories of the single moment when they first thought: this will be all right, this is going to work. For me it came with an astonishing lurch of the heart, when I realised that among those carrying the Olympic flag was Doreen Lawrence. In a way that had no precedent, the country that I lived in – or perhaps the country I wanted to live in – was fully represented, honoured, understood, with the worst that it could do acknowledged along with the best.

The question remains: why had so many apparently well-informed people misread the signs so badly? As a curtain-raiser to London 2012, the US magazine Newsweek ran an apocalyptic warning of doom and failure (by a British journalist) under the headline: "Drunk and broken Britain." Well, our drunks are still boozing. Forlorn estates still breed crime and despair. No one has abolished double-dip recession with an Olympic-branded magic wand. Austerity Chancellor George Osborne – in that historic proof that the Games goodwill did not make the happy crowds take leave of their senses – endured the stadium boos. Those Paralympian over-achievers still face deep cuts. But the kneejerk, sneery American media that looked forward to disaster have utterly ignored the Paralympics: surely the summer's key breakthrough, as both a testament to evolving attitudes and a harbinger (let's hope) of good things to come.

All the intractable divisions of British society remain. Still, from the time in May that the Olympic flame began to pass from town to town through always-cheerful crowds, it became clear to any unbigoted observer that most people both wanted to enjoy the show, and were prepared to make it succeed. Moreover, as soon as the competition kicked off, we could see the somewhat camp, music-hall patriotism that cheered along the British athletes left room for a grudge-free appreciation of everyone else's talent and success. Yet on left and right alike, the culture war declared by the British pundit class on their fellow citizens – an ugly blend of snobbery, prejudice and downright ignorance – had closed minds and skewed perceptions, chronically underestimating the people's power to make these Games. Hence the G4S fiasco became not a glitch but a defining symptom of looming catastrophe. From Martin Amis to David Starkey, this lip-curling scorn for the messy, hopeful actuality of British life has ceased to operate as any kind of positive stimulus to reform. Rather, it serves instead as a pillar of the deeply reactionary ideology of "declinism". Could the Games kill off such declinism for good? We should hope so.

I met the novelist Ian McEwan in a sun-baked city two days before the Olympics ended. "This is the first time in the national narrative that I remember when people have actually said, we are living through good times," he noted. "I've never known London in such a good mood. I've never spoken to so many strangers. It might just be a weird delirium, like something out A Midsummer Night's Dream – we'll wake up next week and notice that the Coalition is falling apart and the trade deficit is the largest on record. It is a curious moment."

Among Shakespeare's plays, it has been The Tempest – which bounced around the Opening Ceremonies – that has given the two Games their mottos and motifs. But A Midsummer Night's Dream may offer a handy source of illumination too. As the mad night of bliss, hopes and quarrels ends, Hippolyta finds a reality behind its shared dreams, or delusions. "But all the story of the night told over,/ And all their minds transfigured so together,/ More witnesseth than fancy's images/ And grows to something of great constancy;/ But, howsoever, strange and admirable."

Will the Olympics in retrospect amount to no more than "fancy's images", or might its meaning grow into "something of great constancy"? That's up to us. The play has ended; the work begins.