Happiness as the key principle in society is the basis of the moral theory of Utilitarianism.
In brief, utilitarianism is a consequentialist theory that is based on the idea of maximising pleasure and minimising pain for the greatest number of people. As a consequentialist theory, it asserts that the consequences of our actions define whether the action is morally right or wrong. For Utilitarianism, a moral action is one that benefits the majority.
The Utility Principle was defined by Jeremy Bentham, a moral philosopher, as 'the greatest happiness for the greatest number'.