The carriages were packed with bodies and the suffocating smell of urine, beer and sweat. As the police shoved their way in, sleepy faces lit up with fear and babies awoke. I followed the operatives, notebook and camera in hand. The passengers seemed prepared for such an event: those with papers legitimising their presence retrieved them from envelopes and plastic bags and offered them up; others capitulated and were led into a corner so that they could be  put off at the next station and processed for repatriation. One young man argued violently with a detective, refusing to acknowledge his nationality, though vaccination marks on his arm seemed to indicate he was Mozambican. I lifted my camera and aimed it at the melee.

The young man snapped his head towards me, anger coursing across his face.

"Why are you taking pictures of me?" he demanded.

"I'm a journalist," I replied.

"Why do you want to put me in the newspaper?" he asked, his voice now broken, his expression turned to grief.

I could not answer him. I was doing my job, but this was too flippant a response. As the stinking carriage rumbled along in the dark towards Waterval Onder, where a courthouse was lit up in anticipation of the early arrival of Mozambican "illegals" – 84 of them, in the end – I understood that this man had been stripped of two things: the hope of a better future, and his dignity.

And herein lay an important lesson for me as a journalist, an insatiable traveller and a citizen of the world: photography is a form of theft. It is a forensic transcription of the moments – life-changing, deeply personal, quotidian – that are knitted together in numberless configurations to comprise a person's life. It can suck out from the marrow of people's bones their composure and pride just as easily as it can confer upon them a sort of flattery by stealth. In the eyes of some Aboriginal people, photographs can steal the soul.

The least we can do when we photograph a person, it seems, is to ask their permission first. For as the shutter releases, we cast in aspic an expression, an incident, an emotion which, in the transition from this moment to the next, has already become history. We stamp our own vision, our own interpretation of that moment, into antiquity. We can return to that image again and again, observing it at a remove from its context; we can share it with others, with observers who are ignorant to its nuances and the subtleties that have informed it, who are oblivious to the people and structures that have been cropped from its carefully considered frame.

The photograph, you see, is an incorrigible liar. Even long ago, upon the dawn of photography, when images took time and patience and money and good light to produce, when the photographer was a rare and daring and esteemed breed, there were those who would stage their images, rearrange subjects, remove elements from the frame, manipulate the scene. There were photographers who would capture images in moments of such innocence and spontaneity that the truth of them would only be discovered decades later: Alfred Eisenstaedt's famous V-J Day photograph of a sailor kissing a nurse in New York's Times Square, for example, is said by some to feature instead a dental assistant and a man dressed up in his sailor's uniform and out on his first date with another woman – the one who would later become his wife, and who appears in the picture's background.

Photography is one of the greatest forms of modern communication; implicit in the creation of an image, in the telling of a story, is the expectation that the story we hold before people's eyes is truthful. A compelling photograph distils some aspect of the world so perfectly it is understood at once by all who observe it. It sets a scene, poses a question, evokes an emotion, prompts deep thought, reaches a conclusion. The most powerful of images – Nick Ut's photograph of Vietnamese girl Kim Phuc running naked and burned from a napalm attack during the Vietnam War, Eric Lefranc's shot of a polar bear mother and cub drifting on a shrinking ice floe in Svalbard's Olga Strait – go as far as to challenge people's beliefs.

It is content, then, rather than technique or polish, which informs great photography. And it is curiosity that informs great content. National Geographic photographer Steve McCurry – best known for his iconic 1984 image of the Afghan Girl, Sharbat Gula – told me in an interview last year that the best photographs were those with which the photographer had some special connection.

"I think if you have a keen curiosity, you notice things other people don't," he said.

"As you walk around you observe things that are around you, get into another state of mind, which is really here and now and in the present. You start seeing all sorts of amazing things … [and] because you're paying such close attention to the environment, great things open up."

Yet few photographs these days reflect the curiosity of the photographer, and fewer still escape the polish-and-gloss that seeps across our photographic output like a nasty rash. Utilitarian, fun applications like Photoshop and Instagram have been so thoroughly abused that most of the images we see today are not so much representations of reality but reflections of what the photographer wished they had seen when observing their subject. Like so many of their human creators, 21st-century images are preened and tweaked, sharpened and smoothed, until they no longer resemble their former selves.

It's no bad thing that digital technology and the proliferation of social media platforms have democratised the once-expensive art of photography. This revolution gives us all access to the joy and power of visual expression. But it can be problematic, too, for boundaries are blurred, personal space is invaded (and not just by selfie sticks), and the pictorial conversation becomes a monologue about where I have been rather than what I have seen.

The world is awash with mobile phone-carrying citizen journalists, people not necessarily schooled in the ethics and etiquette of reportage. It is replete with camera-wielding travellers hopping on and off planes, trains and boats, whistle-stopping through other people's countries, entering villages where children play and mothers toil over open fires, walking along cobblestoned streets where freshly laundered underwear has been strung aloft, passing the open doors of jetty-dwellers and the riverbeds alongside which naked people bathe. All the while, their cameras are poised, ready to capture and disseminate the lives of perfect strangers.

This shrinking of the world, coupled with our abiding connection to social media, prompts a philosophical question: if a person travels abroad without taking a single photo, were they abroad at all? Friends and followers would surely say no. For publicity seems to have replaced enlightenment as the chief byproduct of travel. There is no point in summiting Kilimanjaro or cruising the Bosporus or seeing exotic people in exotic lands without simultaneously delivering a rolling report on our excursions. We take pictures everywhere we go; we shovel them as soon as Wi-Fi connection allows into the news feeds of an insatiable, photo-hungry public. This orgy of lens-snapping contributes to the 1.8 billion (and counting) images that appear online each day. Much of this output – devalued by its very ubiquity – comprises the vain and blinkered reflections of our own faces or the benign interactions of social groups; but the other shots we steal – the pictures of strangers, of the details of their lives – demand an urgent inquiry: how do we respect the dignity of our subjects?

The answer to this question came to me from photographer Fredric Roberts, whom I met at the camp we were staying at during the 2013 Maha Kumbh Mela in Allahabad, India. This is the world's largest religious gathering, where up to 20 million Hindus converge on the Ganges, Jamuna and mythical Saraswati rivers every 12 years to perform puja. In an interview, which was later published in an American photography magazine, I asked Roberts how he had approached his subjects. His answer was unequivocal: he left his camera behind. He walked into one of the big ashrams on the main road where sadhus came for their meals each day and sat with them. He returned the next day and the next, until a relationship had been formed. Only then did he start shooting his subjects. It was an approach, he said, that ensured he wasn't "just another jerk with a camera".

"There were 100,000 photographers all taking the same pictures and, frankly, I didn't want those," he told me. "For that I could buy a book."

Sometimes we have moments, not days, to capture the people, the landscapes, the smells, the colours, the vast and ineffable riches we encounter on our travels. Perhaps this is when we should do what Roberts suggests: abandon our cameras altogether. Engage with the people we meet, immerse ourselves in their world, allow the images that bloom before us to sear themselves on to our psyches instead.