Of course dragons lie on gold. Otherwise, how do you think dragons turn green?
Mold.
And I don’t buy that sheer greed <==> gold gathering. Even the greediest being slakes that greed by gathering something that’s worth something to them. Where are dragons gonna spend it, and on what?
PJ: Fell, baby, I thought we discussed this. You’re hungry. You want to munch some man-flesh.
FB: <piteously whining> But humans give me heartburn.
PJ: You don’t really have to eat them Felly, just pretend to eat them.
FB: I don’t know…
PJ: <muttering under his breath> @%!&ing Method Actors!
The Dragons themselves aren’t materialistic, but they are excessively greedy. Part of the function of the Riders is to keep them in check, otherwise they would happily gorge themselves into corpulent obesity. Part of the explanation for this tendency is that they tend to live in the here-and-now, and in a kind of (I always thought) catlike way are only truly motivated by instincts.
Also, on more than a few occasions in the Series, some materialistic Riders cause problems with the Craftsmen and Holders who tithe to them.
None of it is about gold though.
In the Forgotten Realms books (and in D&D worlds in general, methinks), dragons collect stuff for a lot of reasons. I think greed and vanity is certainly a factor; wealth is prestige, and the desire to hoard seems to be innate. There are other practical concerns as well. Some dragons are spellcasters and may need components (such as jewels) for certain spells (though I think some spells are innate). Some dragons command an organization of thugs of various species, and probably need some method of payment. Some dragons can shapeshift to human or elven form, and thus can actually spend their treasure. Dragons collect magical items for their power (to use or trade), and I think I read somewhere that they like to bask in the glow of magic.
In the FR, dragons are pretty pimp 
It’s not dragons, but gryphons were known to protect gold from greedy humans. Much better to flame a few knights than let a whole village or country kill itself over some stupid metal.
The best answer so far has been that the treasure is great bait to attack would-be-hero apperitifs.
Gold makes ideal bedding for a dragon. It’s relatively soft (malleability) and it won’t rust (which makes it good for a long-lived creature). Good nesting material, as well.
Maybe they’re just contrarian investors.
Someone risked life and limb to pass off copper as gold to a dragon.
Ha Smog, I like that, funny
Dragons motivation for hoarding gold? they are basically greedy creatures, they apparently exist to kill heroes steel princess and hoard gold, one might as well ask why magpies steal anything small and shiny or why ocelots hang upside down from trees by a single hind foot, the answer is pretty much the same, a mixture of boredome, the simple fact that they can and because gold and treasure some how appeals to dragons and a level other than its monetary value.
I’ve seen some interesting theories in various books
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Dragons were more of an element of energy that needed metal to go through the stages (only in the last stage did they have physical form). Gold in particular was useful. They had very little inteligence or desires other then finding metals.
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Dragons actually produced treasure. IE copper dragons had blood that became copper gold dragons became gold etc.
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Dragons were obsessive compulsive hyper aware and greedy by nature. So if they were in a field they’d count every blade of grass. Taking gold home was just a way to count that instead of pebbles.