On theism, why do humans have moral duties even if there are objective moral values?

[Angrboda]

I was listening to William Lane Craig's moral argument. Even if we grant him that God exists and objective morality exists and god is the basis of objective morality, why do humans have moral duties? Why do we have to be good and do good other than for a selfish interest in going to heaven? He just claims that we do have moral duties (just like he claims by fiat that objective moral values exist). But why, on theism, do humans have a moral duty to be good?

12 All who sin apart from the law will also perish apart from the law, and all who sin under the law will be judged by the law. 13 For it is not those who hear the law who are righteous in God’s sight, but it is those who obey the law who will be declared righteous. 14 (Indeed, when Gentiles, who do not have the law, do by nature things required by the law, they are a law for themselves, even though they do not have the law. 15 They show that the requirements of the law are written on their hearts, their consciences also bearing witness, and their thoughts sometimes accusing them and at other times even defending them.)

Romans 2:12-15 (NIV)