Simple: when society benefits, you benefit. Your life is better when you are ethical.

It may seem in the short term that you can gain by a pattern of self-interested, unethical behavior when the society at large acts ethically; and sometimes, over the short term, this is true. But the surprising results from the competition I mentioned above is that a pattern of not cooperating with the society in order to achieve short-term gain is not sustainable over the long term; individual entities actually do better on a personal level when their actions are ethical.