Why are we greedy?

This is in response to Nicole B.'s question on one of her posts. The question is :Why are we biologically and psychologically greedy and envious creatures?
My answer is that we are greedy and envious because it makes sense to be. Many people on average, want to be successful. I do no think there is one person that has made it through life without looking at something and wanting it, whether that something was a new car or a better figure. We want things because we believe that if we have them, we can lead a happier life. Commercials do not help us with this issue. In advertising, you see a person running on the beach with all their friends, and then as their laughing and having a good time, you see one of them pick up a drink. the whole message this commercial conveys is that if you buy the drink you can be happy like the people in the commercial. Many people in society may not be able to afford the drink, but that does not stop them from wanting it. I think that in some cases being "greedy" is healthy. Greedy being defined as wanting something. When you want something bad enough you are motivated to get things done to get that something.
question: Can greed be a good thing?