Dragon Treasure
Not all dragons collect treasure - Oriental dragons in particular tend to be above such mundane worldly concerns. However Western dragons are famous for collecting and sitting on great piles of gold.
So why this link between dragons and treasure? After all, they can hardly wear or spend it!
It probably goes back the the image of the Western dragon as an evil creature. The treasure hoard was a symbol of the dragon's greed and materialism.
The great treasure also provided a valuable reward for the brave knight, hero or adventurer who faced and slew the dragon. Of course there was a danger that in gaining this great wealth the dragonslayer would himself become corrupted by it. In particular those who sought out dragons specifically for their gold rather than for more noble motives could be seen to have already been corrupted by materialism. This may be the origin of some of the stories of people turning into dragons. Eustace from the Narnia saga is a good example of this.