Hardin, G. (1968). "The Tradegy of the Commons." Science, 162(December): 1243-1248.

In many situations the self-maximizing gains by individuals ultimately destroy the resource, such that nobody wins. The utility of an additional item for the person is more than the disutility of the additional strain on resources (shared by everybody).

This occurs with park usage, pollution, etc. Appeals to conscience will not be effective. In a sense we need to restrict freedom to gain it.