Because the dragon had taken the treasure from the dwarfs, so they wanted to reclaim as much of it as they could.
Answer:
Tolkien's dwarves are patterned after the Germanic and Norse dwarves. It is the nature of such dwarves to want as much treasure as they can get and keep it, this is much the same as the nature of dragons. Unlike dragons who saw the possession of gold as a good thing in itself and mostly sat and slept on their huge hoard, dwarves were skilled metal workers and viewed the treasure they hoarded as valuable for both its intrinsic value and its role as a raw material for their metal working. The magic of gold and jewels were essential for the amulets and weapons they produced.
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J.R.R Tolkien writes "Bilbo had heard tell and sing of dragon-hoards before, but the splendour, the lust, the glory of such treasure had never yet come home to him. His heart was filled and pierced with enchantment and with the desire of dwarves; and he gazed motionless, almost forgetting the frightful guardian, at the gold beyond price and count" From this we can summarize that Bilbo was absolutely taken aback from the wondrous sights he sees. J.R.R Tolkien also writes that Bilbo gazed upon the dragon and treasure "for what seemed an age" in my opinion he felt a certain thrill from this sight. Hope that helps!
To Reclaim their stolen gold from Smaug the Dragon.
Bilbo told Smaug a riddle to keep him distracted.
Though it seemed wise at the time Bilbo took the cup, it was actually very unwise, because a dragon can sense when a part of it's treasure is missing, and will not rest until it has killed the thief. This is what made Smaug aware of the dwarves' presence to begin with.
The biggest treasure was the accumulated wealth of Smaug. The Arkenstone that was a part of it was greatly desired by the dwarfs. There was also some treasure from the trolls, and Bilbo found the One Ring.