Smaug is the embodiment of earthly materialism. The treasure that he hoards is not his own - it is material wealth gained from the people that he has conquered. He is just a step away from those people, who worked so hard to gather this wealth. The battle that ensues upon his death just goes to show this - his "fire" is the passion that men and dwarves and others have for wealth, and it consumes them as much as Smaug's fire consumes them.
Tolkien himself calls attention to the symbolic nature of Smaug when discussing Smaug's anger:
"is only seen when rich folk that have more than they can enjoy lose something they have long had but never before used or wanted."
Tolkien mentions "rich folk" and implies that the wealth of these folks is useless in nature. Tolkien reinforces the symbolism of Smaug as representative of humanity by giving him so many human traits. Smaug's personality is captivating; he has a penchant for riddles (a very human thing); he is arrogant. The only way to really tell the difference between him and the species of men is in his dragon body.