Dragons and Treasures

As I was sitting in class earlier, trying to get through today chapter of "Silmarillion"-Tolkien, it came to my mind that whenever you read a story with a dragon in it, there's always -ALWAYS- a treasure involved. If the dragon's protecting a tresure, hunting one down, stealing it, whatever it is, there's always a treasure.

Look only at these books; "there and back again"-Tolkien, the dragon's guarding the rich treasures he stole from the dwarfs decades ago, sleeping on them like a golden princess' bed. Harry Potter two books, "the goblet of fire" where the dragons are guarding the golden eggs and "the deathly hollows" where they are used as "watch dogs" in the deepest, most valuable parts of Gringotts. "Narnia; the voyage of the dawn treader"-C.S. Lewis, the dragon was once a man, who got greedy when he saw the gold and jewels, and turned into a creature fit for his greed by the cursed treasure.
And just to have it added, the old folktales, like in "Beowulf" (the epic poem) the famous king is fighting the dragon to take back the treasures it had stolen.

Now, I also noticed that every other mythical creature has something to follow it, like the dwarfs are always short, elves have pointed ears and so on. But it usually points to the features of whatever it is, no other creature is that haunted by a THING. So my question; what is it that makes dragons so bound to treasures?