If there is no god, why do people behave in a moral and ethical manner?
One answer might be long-term self-interest: if you never tell a lie, for example, you will develop a favorable reputation among other people which will allow you to participate in all sorts of activities of which you would never be a part otherwise.
Another answer might be "big picture" self-interest: people usually achieve more and have higher standards of living when they collaborate compared to when they compete: "competition" only works as a motivator when embedded in a broader collaborative structure first (i.e., if everyone plays by the rules, we aren't deliberately trying to injure a competitor because we don't want them trying to injure us and so we all place voluntary limits on our behaviors to promote a better outcome for all).
While these answers are all well and good, there seems to be something missing: to be motivated SOLELY by self-interest, no matter how you dress it up, seems like a somewhat barren life. ...