Because humans are highly social animals that have used cooperation and mutual support as a survival strategy for millions of years. When catastrophe strikes they are forced to band together in ways that modern society doesn't usually allow for. This sense of camaraderie, shared purpose, and taking care of each other is what humans are designed to do. It's in our genetic wiring (according to scientists who have studied this). Being in a war in a city that isn't pushed to these levels of cooperation is just depressing and isolating, but if you lived in London or Dusseldorf, that extra layer of chaos from being bombed meant that people were more likely to band together in the ways that feel good to humans.

It's part of why there is so much loneliness in modern life. We aren't designed to live as individual islands, with only a few others for support. There are times where scarcity of natural resources drives competition and violence, but in many cases, people really do have a natural inclination to cooperate and share. After all, that's the survival strategy that got us 97% of the way to where we are today.