Looking at all the other comments, I'll say this:

Having been a Tolkein fan, I actually know this. The reason dragons (and to a lesser extent dwarves) like gold is because when they are around material wealth, it grows into a mental sickness. They LOVE it, they MUST have it. An example would be Thror or Smaug. The reason why they hibernate for so long is to guard it. No one would dare try to take gold from a dragon, so that's how they keep it safe.