Because human beings have not ended. Make all advantages given to ourselves. Has no respect for the rules and make it more difficult to control, it is possible the same effect occurs when you are together.
Because humans have an infinite demand. Everyone is looking to make himself useful. Do not respect the rules and control the possible difficulty over for good when placed together.
Because humans have needs, endlessly. Make everyone exploits for themselves. Born not respecting the rules and make control more difficult when possible good together.