Places change. Towns grow and cities sprawl. People like to be near other people. Industries and businesses like to huddle together to maximise their opportunities. We live together, socialise together, work together and grow together. That's the nature of life today. We like cities and we like city life.
Belfast is often described as a "divided city", yet, in many ways, it is no more divided than most other cities - be they the capital city of a country, or just an important regional hub.
In New York, London and Paris, for instance, opulence and poverty rub shoulders together across a few square miles. These are us-and-them cities, too, albeit an us-and-them based on social class rather than religious/political differences. There are areas in almost every major city that are run by gangs of one sort or another and into which the forces of law and order rarely go.
Most cities are, in reality, a collection of villages and self-contained areas, complete with their own very distinctive habits, culture, lifestyle and way of life. Leaving one huddle of streets for another is like leaving one country for another.
People may say they live in London, New York, or Paris, but often they live in a part of the city that wouldn't even be recognised by their supposed fellow citizens. They have different incomes, different houses, different schools and different standards.
So, in one sense, Belfast is, in fact, pretty much the same as most other major cities. But there is one significant difference: Belfast has areas which are almost exclusively Protestant/unionist, or Catholic/nationalist and which, in turn, are further sub-divided into the traditional social classes.
So, while it has the same sort of socio-economic differences you would get in other cities, the classes will still prioritise their "own sort", rather than the "other sort". And that also explains our political parties and institutions.
Cities change for a number of reasons. In some cases, it is because of the rebuilding required after the devastation of war - as was the case for many cities in 1918 and 1945.
The rebuilding will often reflect post-war realities ("building a country fit for heroes"), as well as providing the opportunity to deal with pre-war problems like housing, sanitation, infrastructure and public transport.
Cities will often change to reflect broader socio-economic challenges, like a booming economy, a demand for skilled workers (who will want better houses and choices to accompany their higher salaries and expectations) and the rise in immigration that followed the end of the Second World War and the expansion of the European Union.
Some of this change will be controlled (the building of Craigavon, the M1 and the Jordanstown project in Northern Ireland, for example), while others can be kneejerk and hurried - bringing problems further down the line.
Sometimes, like Belfast, a city needs to rebuild and reinvent itself as a signal to the world - tourists and investors in particular - that it has changed.
Belfast has been doing that since the mid-1990s (although it has had to cope with the physical damage inflicted by terrorism since the early-1970s). What isn't entirely clear at this point is whether the city is changing beyond the architecture. Are we building a shared city which will allow us to avoid the collective errors of our past, or are we merely putting up a good "front" to fool those tourists and investors?
That's why the photographs released by the Public Record Office of Northern Ireland (PRONI) this week are so fascinating and so relevant. Yes, they indicate change, but maybe it's the change described by Jean-Baptiste Karr - "the more things change, the more they stay the same".
Sam Cromie from PRONI puts it like this: "It shows how much things have changed in the past hundred years, but also how much they have stayed the same. The streets and buildings are still there for the most part and it's still Belfast, but the life around them is almost unrecognisable.
"Looking at the photographs, the biggest change is transport. Boats and trams have been replaced by cars and buses, and the infrastructure of the city has evolved with this. Cobbled streets have given way to roads and tram tracks are now bus lanes."
In one sense Cromie is right; the infrastructure has changed and it will continue to change. But is life around us "almost unrecognisable"? I'm pretty sure that, if you were able to transport people from 1915 to 2015, they wouldn't detect much of a change in Belfast itself in terms of how we deal with each other.
We are still a divided city, still a city in which we mark out our own territory with murals and street signs and schools and churches and even leisure centres. We have the Waterfront Hall, the Titanic Centre, Victoria Square, the Odyssey Centre, a revamped Lyric Theatre and Ulster Museum: we even have a brand new building for the Public Record Office. But we still have the "dreary steeples" mentality at the very heart.
In their 2005 book, Made in Belfast, Trevor Parkhill and Vivienne Pollock wrote: "In 1900, Belfast had the world's biggest linen factories and the largest ropeworks. Its shipbuilding industry produced more tonnage than any other city.
"But Belfast was also home to cycle-makers, printers, artists, tea magnates, clog-makers, clothiers, photographers, engineers and tobacco manufacturers."
Belfast was an important city, a world-class city, and an industrial jewel in the old British Empire.
Yet the Belfast trade unionist, Robert McElborough (who lived in Sandy Row, worked on the trams and in the gas industry and was an active organiser in two loyalist trade unions), writing his memoirs of life in Belfast in the opening years of the 20th century, said: "My experience of living in rooms with nothing to eat only what friends gave us is engraved on my memory."
And that's probably the one thing that our visitor from 1915 would notice about Belfast. The workhouse has gone. The grinding poverty has gone. Houses, even in the poorest areas, have inside toilets and baths. Children can stay on at school and the brighter ones - irrespective of background - have opportunities that would have seemed unimaginable a century ago.
But the Sandy Row that McElborough knew in 1915 is still marked out as a "loyalist" area and he would still have a role to play in uniting the working classes from both sides. The work he was involved in then remains unfinished business.
Sandy Row may have new, warmer houses and it may be a few hundred yards from the M1 and a bustling city centre, but it is, by and large, the Sandy Row of a century ago: albeit reduced in size and influence within loyalism and broader unionism.
There are council wards across Belfast - in both unionist and republican areas - which have some of the worst levels of education, health, employment and mental illness across the United Kingdom; and those are problems that need to be tackled with the same sort of enthusiasm shown rebuilding our city.
A city isn't just a physical thing of bricks and mortar; it is also a community thing, a collective thing, and a psychological thing. A city is as much about its arts and culture as it is about its award-winning buildings. It's as much about the collective identity of its citizens as it is about award-winning promotions about "come to Belfast and see the changes".
The fact that so many of the PRONI photographs from the 1900s can be blended so easily with photographs taken a few months ago tells you something very specific about Belfast: physical change, like socio/political change, comes "dropping slow". Maybe that's a good thing, because most of the evidence suggests that we don't like things to change quickly here. I'm wary of social engineering - although I suspect that a lot of architects and politicians quite like the idea. That said, cities do have a role to play. They reflect who we are.
The very diversity of a city, with its mini-villages, ethnic areas, arts, cuisine, culture et al, tells you something about the multiculturalism that underpins the place.
It tells you something about the ability, willingness and determination of people to promote their own identity, while recognising and respecting the identity of others. Sometimes it boils down to something as simple as "yes, we can live together and intermix very happily".
Has Belfast reached that point yet? Well, the city certainly looks very good. It is full of tourists who seem to love the place. It has a bounce and zest it has never had before. And, yet, there is also a sense that Victoria Square, the Odyssey, the Lyric and the Titanic Centre are regarded as "neutral areas" by the locals.
We like to hear them talked about and admired - and we even go to them. But then we return to our own areas and schools and churches and activities. We may be happy to share our city with others from across the world, but we still seem to have difficulty sharing it with someone from a mile down the road.
Belfast has changed. It is a much, much better place to live in than it was a century ago, or in the 1960s, or even the 1980s. We do have some of the best housing in the United Kingdom (or south of the border), a waterfront that is just wonderful, a creeping Bohemianism and a genuine sense of feelgood about ourselves.
But we need to ensure that it isn't all just a mere front after all. Our city is something to be proud of: still standing having endured so much. It's about time we learned to share it.