The normative power of ethics is not an independent fact. Ethics is normative because of our other desires, because acting ethically helps us achieve those other desires. The reason this is important is because of a certain popular misconception about the normative force of ethics. Specifically some think that if ethics gives you a reason to do or not to do something then that reason rightfully outweighs all other considerations; that if ethics tells you not to do something then there is never a case in which you should do that thing. I think this is wrong.
The proper view, in my opinion, is that ethics gives us a reason to do and not to do certain things. But our other desires give us reasons too. Thus ethics is but one source of reasons among others, and thus there are times in which we will have reason to go against the recommendations of ethics, just as at other times ethics gives us reason to go against the recommendations of our desires. But for some reason people tend not to consider this possibility; many approach ethics as though the only alternatives were for the recommendations of ethics to be absolute or for the recommendations of ethics to have no normative force. I see this as a false dichotomy. Certainly there are many other situations in which something is normative, and yet which we may rightly choose to ignore on occasion. For example, going to sleep at a reasonable time is normative, but on occasion we may choose not too, in order to satisfy some of our other desires.
I see this middle position, viewing ethics as only one source of reasons among others, as the most compelling because of the source of the normativity of ethics, which I mentioned above. Namely that we have reason to act ethically because in most cases acting ethically is the best way to further our desires, at least in the long-term. But there may very well be cases in which the best way to satisfy our desires, in both short-term and long-term, will be to act unethically. In such cases we may have reason to act unethically. And, besides seeming to follow from an understanding of the source of the normativity of ethics, there are other reasons to favor this view. For example, it resolves certain ethical conundrums, such as why it seems right to prefer the well-being of the people you care about over the well-being of people who are strangers to you. The traditional response has been to try and show how such favoritism is really the ethical thing to do, or at least ethically neutral. But if we understand ethics as simply one source of reasons then we can deal with these situations by admitting that ethics gives us reasons to treat everyone equally. But we can also admit that our other desires give us reason to favor those we care about, and that these reasons override the recommendation of ethics to treat everyone equally.
Of course we also have to factor into these considerations the fact that different people are motivated by ethics to different degrees. Now everyone has reason to act ethically, regardless of their desires. But some people, in addition to this, desire to act ethically for its own sake. And this is a desire that has different force for different people. Obviously the more a person desires to act ethically the fewer situations there will be in which they have reason to ignore the recommendations of ethics. And, in addition, this explains why some people are willing to give up their lives due to ethical motivations. If a person only acted ethically because it helped to fulfill their other desires then they would never follow an ethical recommendation that would lead to their own demise, because you certainly can’t fulfill any of your desires when you are dead. But if your desire to act ethically is strong enough you could be led to follow the recommendations of ethics in such a situation, because by dying you would be fulfilling your desire to act ethically.
Of course this leaves the question open as to how much weight ethics carries in comparison to our other desires. That is not a question I wish to address here. I merely want to point out that putting ethics on a pedestal, treating its recommendations as overriding all others, is a mistake. Holding such a view would only lead to problems, because if you actually followed it you would at times frustrate yourself unnecessarily, and would be engaged in the kind of internal contradiction that prevents people from leading the good life. And your ethical theories would always fall short, because you would never be able to justify a system of ethics which had that strong of a normative force.