Tolkien built Smaug very cleverly - since The Hobbit, many tales about greedy dragons appeared and all were ending badly for the beast. Mostly, because those dragons were a metaphore of human pride and greed, which is gratifying for some short time, but in the end always turns into something disastrous.

Now, pride is not something totally bad, as long as it doesn't close person's eyes on reality, building false view on themselves or make them harm others. Greed actually is straight out bad.

People with these characteristic are very often rich beyond comprehention, always wanting more, stepping to success on backs on others.

And just as Smaug got an arrow in the heart, people like such, are never truly fulfilled, and in the end, put money and vanity over own life, building empires made of paper and drowning in own riches, in luxury cages.

A greedy dragon may be a caricature of those humans. I personally though, love these great beasts. Their human counterparts - not so much. It must be this shine to those scales (haha!).