I basically take the Epicurean position... humans' greatest pursuit in life is happiness, and the greatest long-term happiness comes from a life of moderation. But I admit that doesn't really answer Craig's question, which is: why not harm others when it would lead to our own happiness?
The fact is, there is no answer. Ethics cannot answer the question of why we should be moral, any more than mathematics can answer the question of why 1+1=2. In both cases, we just have to start with an axiom (that harming others is bad, that units can be aggregated) and build from there.
People who do not accept this basic axiom (that harming others is bad) are called sociopaths. That is because the intuitive acceptance of this principle is part of our human nature. Its nonacceptance is considered a breakdown of that nature, an illness.
But there is no reason why you should avoid harming others, any more than there is a reason why you should want to be happy. They're both just part of the way we're wired.