When the prospect of getting something for nothing comes up, most people naturally tend to jump at the idea. If you can get your hands on more of something desirable than another person, you will naturally go for it.
Humans tend to lean toward greedy because of one simple fact. They can.
Do you think you are naturally greedy? If given the opportunity, would you try to take more money, food, gold, diamonds, whatever if you had the chance? If you had the chance to make more money at the expense of another if you owned and ran a business, would your natural tendency not be to try to make it?