It's in man's nature to watch out for himself before anyone else but greed (a selfish or excessive desire for more than is needed or deserved, especially of money, wealth, food, or other possessions) is not part of the human nature. Greed and selfishness are traits that are developed over time.
For most people, greed develops as a result of fear of loss or as a result of pain suffered when they were a lot younger. Freed becomes a survival technique for these people. Some people are just greedy by nature, the good thing about this is that, character can be learn and unlearn. A greedy person can work on himself or herself to become generous.