Why did many cities have such problems with fire? What problem with housing made it worse?

In old times, houses were mostly built out of wood or just had a lot of wood which was an important element out of which houses were made. For this reason, if something was set on fire, houses could easily catch fire. Another thing to note was that streets were very close and narrow, this meant that houses were positioned very close to each other.

Both of these things influenced the rate of fires in old towns.