That said, it should remind us that everything -- no matter how solid and ancient and enduring it seems -- is fragile. All of life and all the artifacts of man and nature are precious, one-off things. Alas, like so many things, you don't appreciate it until it's gone.
Fire was dreaded in the Middle Ages and a cathedral burning wasn't unknown, especially when the only source of light was fire from candles or torches. And cathedrals aren't just stone. There's a lot of hangings and wooden furnishings -- much of it very old and dry. The roofs are very old, with huge timber frames, and are covered in lead, which has a low melting point. Like a canopy fire in a forest, once the flames reach the roof, the huge, hollow structure of the Gothic cathedral acts like a giant oast. And when the fire gets hot enough, limestone will burn, too.