Look at a city from above — a million lights, ten thousand streets, people stacked in towers fifty stories high. Why would anyone choose to squeeze together like that when there's so much empty space everywhere else?

Turns out humans are like magnets that only work on other humans. The first person opens a bakery. The second person needs bread, so she moves nearby. The third person sells flour to the baker. The fourth repairs the oven. Each new person makes the place more useful for the next.

This snowball effect has a fancy name: agglomeration. It just means "good stuff piles up in one spot." A city with a hundred restaurants can support a spice importer. A city with ten hospitals can support a company that only fixes MRI machines. Small towns can't — there's not enough business.

Jobs pile up the same way. If you're a software engineer in a small town and your company closes, you might have to move. In a city, fifty other companies are hiring the same skills within a ten-minute subway ride. Cities are insurance against bad luck.

Then there's the weird magic of ideas bumping into each other. A game designer overhears a biologist at a coffee shop talking about how ants communicate — boom, new game mechanic. A chef smells the spices from the restaurant next door — boom, fusion menu. You can't schedule that. It just happens when people mix.

Cities also split up the cost of expensive things. One library serves a hundred thousand people. One subway system moves millions. One power plant, one water treatment facility, one fire department with fancy equipment — everyone chips in through taxes, so no single person pays for the whole thing.

Of course, cities have trade-offs. Your apartment is smaller. The subway smells like a thousand strangers. Rent costs more because everyone wants the same hundred square miles. But for millions of people, the math works: more opportunities, more weird encounters, more kinds of pizza.

So people live in cities for the same reason trees grow close in a forest — they're better together. Every person makes the system a little more useful for everyone else. It's not about the buildings. It's about the density of human possibility.