I'm not sure that greed is the right word to use here. I think everybody wants to get more money. The mistake is being careless. This often happens when you let emotions get involved. If you get upset after losing some money, you might make a bad decision afterward. You may also illogically think that if one thing worked once, it will work again. You need to diversify. You can't put too much of your money in one place. That's a recipe for failure.