Every moral law has two components: substance and imperative. The substance is the nature of the moral quality involved (i.e., goodness, nobility, courage, etc.). The imperative is the obligation to exemplify that quality. Even if a moral theory could explain the origin of a quality, it still falls short unless it also encompasses the imperative. The mere existence of a quality does not obligate people to emulate it. Therefore, an adequate moral theory must explain not only the substance of morality, but also why people are under any imperative to act in accordance with that substance.
Most theories cannot justify the imperative without begging the question and (at least implicitly) using an imperative in their reasoning. For example, evolutionary theories claim the moral action is the one that preserves society. However, “This will preserve society cannot lead to do this except by the mediation of society ought to be preserved,”  which is circular reasoning. Aristotelian theories argue that the moral action is found in the mean between two extremes.  But again, there can be no obligation to perform the action found in the mean unless we assume an imperative that we ought to model the mean.
This is one of the advantages of divine command theories. Properly formulated, these theories provide an explanation for both the substance and imperative of moral obligations. Their substance is found in God’s character. When an individual person is described as “honest” or “empathetic” those are general descriptions at best. After all, nobody perfectly exemplifies these character traits all the time, but they may be more present in some people than in others. These same qualities, however, exist perfectly in God. He is always honest and empathetic. To say that someone is honest is to say that they act in a way that is similar to the quality in God’s character that is described as “honesty.” God’s character is the source for moral descriptions. It provides the substance. A description of the substance is as far as most moral theories can travel. But divine command theory also includes a description of the imperative. We are obligated to model our behavior after those traits in God’s character because he has commanded us to do so. Therefore, divine command theories address both substance and imperative.
If the imperative of moral rules lies in God’s commands, a critic could justifiably ask why people should obey them. Just because someone orders you to do something does not mean you should do it. If Aristotelian ethics fail to explain why people should behave according to the mean between extremes, do divine command ethics have the same shortcoming? Instead of failing to justify why people should obey the mean, they fail to explain why people should obey God.
I have begun to formulate a possible answer. First, the very nature of a command raises an issue of obedience. The intention of a command is for it to be obeyed. The same cannot be said, for example, of a mean. There is no intelligence behind a mean expecting obedience. Therefore the mere fact that a mean exists does not raise the question of taking action in compliance with that mean.
At the very least, then, basing an ethic in commands legitimizes an imperative whereas other ethical theories do not. However, not all commands are to be obeyed. The command of a would-be murderer to assist in a killing is properly disregarded. So while it is true that only a command can legitimize an imperative, the question remains of why God’s commands are of the nature that they should be obeyed.
My preliminary answer to this dilemma lies in the fall. All humankind was made in the image of God. The positive attributes of their character were passed on from God in a similar fashion as a parent passes on genetic qualities to biological children. The fall, however, tore several holes in the character of the human race. People may have only been “good” in a finite sense prior to the fall, but afterward they became dramatically less so. Yet the image of God did not abandon humanity altogether. Programmed into humanity’s very being is an irresistible longing to return to that original state. It is a goal that people cannot help but desire, even if they cannot adequately recognize or articulate it. Humankind knows something is missing, although they may not know what it is. A gap longs to be filled, and a gap in moral character is no different. God’s commands tell people how to fill that gap as much as is possible in this fallen state and therefore satisfy this longing. If they recognized that God’s commands pointed toward this goal for which they irresistibly strive, the conclusion that God’s commands should be followed would be equally irresistible. In this sense, “should” is not a moral imperative, but rather a necessary presupposition based upon a fallen nature.