This emotion is related to the feeling of entitlement and belief that we deserve a reward, especially when we have a comparison point. When we see our friend buying a new TV we suddenly want one as well, maybe even a better model. In the end, we also work hard!
Greed is what makes us continuously wish to have more and more and more. For the investors, greed pushes them to buy the stocks when the price is on the rise, which causes artificial price bubble ready to burst any time sending most of the greedy ones home with empty pockets.