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

[Pnerd]

Most replies here answered some other question.

The primary answers given were a) doing good is rewarding in itself (both natively...

Theism isn't necessary for that. That's why I don't find that a useful reply.  

... and because God has made it so [Angrboda, Mister Agenda])

Do you mean God has made the rules that doing good would result in reward in the afterlife? The last part of my question was "other than for a selfish interest in going to heaven or avoiding hell". "God has made it so" (if it means doing good would result in divine reward) seems to be about god's reward and punishment as a motivation for theists to do good, which is something my question wasn't/isn't looking for in an answer.

... and b) Natural law makes being good rewarding (because God has made us so that doing good is natural and unproblematic [Belacqua]).

I probably missed or misunderstood this answer previously. This one makes sense.

If I were to guess...

Please don't. It's a shocking habit; destructive to the logical faculties. 

... you're looking for an answer with specific properties (it fits your preconceived ideas, it answers some previous argument, it satisfies you intellectually, etc.), none of which actually follows from the question you asked.

You can ask instead of guessing.

It could be that I didn't manage to communicate my question properly (English isn't my first language). 
.