Otherwise it would be a tautology to say 'God enjoins what is good' because it would only mean God enjoins what He enjoins. But the Qur'an abounds in statements like the former, and it is very clear that they are not intended to be tautological. The answer to our original question then is that we do what is good because it is good. But since to give good for good is itself good, there is no contradiction in saying that one does good because the God whom he loves and in Whom he puts his trust tells him to do it, and because he expects to be rewarded by Him for doing it. True religion is built on the basis of this original human nature.
Religion strengthens nature and brings to fruition the seeds of virtue that reside in it.
That is why Islam is said in the Qur'an to be fitrat-Allah and why the Prophet says that he was sent only to perfect good conduct. Those whose striving goes astray in the present life while they think that they are working good deeds. But since a Muslim surrenders himself to God and loves and fears Him, and since God loves virtue and enjoins it and hates vice and forbids it, he does the former and avoids the latter in obedience to his Lord. So after that let the strivers strive. These in resume are the basic truths to which the Prophet Muharnmad invited his people.