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

[Belacqua]

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?

I don't know what William Lane Craig says. He gets a lot of attention these days, but I don't find him to be a very interesting thinker.

There's a lot of variation among Christians, of course. So you'd get a variety of different answers if you went to a Christian site. I can outline the view that I studied, when I was reading Christian theology:

~ Human beings are a certain kind of animal, with particular needs, abilities, limitations, etc. 

~ There are ways of behaving that are suitable to our particular qualities, and ways that aren't. The best behavior is that which aims toward our best overall well-being. 

~ We may not understand what is best for us in any given situation. This is why we have various institutions to provide guidelines. (Though of course institutions may fail to know well.) 

~ Humans are by nature social animals. No one is an island. One's overall well-being depends very much on the well-being of others. 

Morality and ethics are the norms we use to aim toward what is best for us, which includes what is best for everyone else. 

"Moral duty" means the recognition that it is bad to harm ourselves or others, and the commitment not to do so. It means adopting the responsibility of behaving with the well-being of ourselves and others constantly in mind.