But one might wonder why we should care about this particular end. If it's completely arbitrary, then moral obligations would seem to lose much of their force. One might as well choose to make evil-doing their ultimate end, and from that perspective one can just as well criticize the altruist. So the big question is: do we have any reason to prefer the ends of morality to those of its opposite?
I think we do, and I hope this isn't just wishful thinking on my part. Though in my case, at least, it doesn't much matter. Fact is, I do value this end, and I think many other people do too, and that in itself is enough for us to care about morality. But on to the larger question...
There is an obvious sense in which morality is in everyone's interests, since the achievement of those interests (i.e. of everyone) is precisely what morality aims at! If everyone behaves morally, then we're all better off than we would be if no-one was moral. The moral perspective is one of social rationality (rather than mere individual rationality), and we need to appeal to this view in order to best resolve the prisoners dilemmas and other collective action problems that otherwise arise. Put another way: we're all better off if we live in a more moral society, so there's a shared 'common interest' we can appeal to here. Just as individual rationality leads us to care about our self-interest, so does social rationality lend reason to care about morality.
To see how this works in more practical terms, note that I've previously argued that morality plays a complementary role to law. Our actions are caused by the interplay of our beliefs and desires. Laws change our beliefs so that we realise that harming other citizens won't help fulfill our desires (because you might get caught and sent to jail). Morality tries to pre-empt the need for this by changing people's desires, so that they prefer to do no harm to begin with. So, insofar as we have reason to care about law, we have similar reason to care about morality. Both are recommended by social rationality, as means to the achievement of our common ends.
This parallel makes it particularly clear why we should care that others behave morally. It's not so clear that individuals must necessarily care about their own morality, however. I don't see that as a major problem though. I can concede that a scoundrel may have no (individually rational) reason to behave morally, without that diminishing the importance of morality more generally. Even the scoundrel should agree that it's best for all involved (including himself) if others are moral; and he should even agree to civil/political measures that would help promote the formation of moral character generally - even in himself. If the measures caused him to acquire a more moral character, this 'cost' (in individually rational terms) would be more than offset by the benefits of having more moral neighbours.
Further, to quote Railton in 'Moral Realism' (from Facts, Values, and Norms, p.31):
[G]ood, general grounds are available for following moral 'oughts', namely, that moral conduct is rational from an impartial point of view. Since in public discourse and private reflection we are often concerned with whether our conduct is justifiable from a general rather than merely personal standpoint, it therefore is far from arbitrary that we attach so much importance to morality as a standard of criticism and self-criticism.
The existence of such phenomena as religion and ideology is evidence for the pervasiveness and seriousness of our concern for impartial justification. Throughout history individuals have sacrificed their interests, even their lives, to meet the demands of religions or ideologies that were compelling to them - in part because they purported to express a universal - the universal - justificatory standpoint. La Rochefoucauld wrote that hypocrisy is the tribute vice pays to virtue, but 'hypocrisy' suggests cynicism. We might better say that ideology is the respect partisans show to impartiality. Morality, then, is not ideology made sincere and general - ideology is intrinsically given to heartfelt generalization. Morality is ideology that has faced the facts.
So, I've argued that we have reason to care about morality because it is a means, recommended by social rationality, to our common ends. Another issue I'd like to consider is whether morality is merely a means. In other words, is moral behaviour intrinsically good (does it 'add more goodness' simply in virtue of being moral?), or is it merely good insofar as it brings about non-morally good effects (such as fulfilling some person's desires)?
I think it is merely a means. Suppose we have a moral obligation to keep our promises. I might then increase the quantity of moral behaviour in the world by promising to do things that I intended to do anyway. ("I promise to breathe in... I promise to breathe out...") Clearly there is nothing particularly good about such behaviour.
Whether evildoing is intrinsically bad might be more controversial. Derek Parfit (in Reasons and Persons, p.47) asks us to consider a case where you have a chance to prevent either a murder or an accidental death (in a forest fire, say). Suppose that you are slightly more likely to succeed at preventing the accidental death. Also, suppose the murderer is about to die soon afterwards anyway, so concerns about any consequences for him are irrelevant to the case. Who would you try to save? Most of us would probably choose the accidental victim. But if evil-doing was intrinsically wrong, then the importance of this would surely outweigh the slightly improved chance of success in the forest-fire-rescue. So, if your intuitions here agree with mine and Parfit's, you would seem to be committed to also agreeing that the mere 'evilness' of an act does not itself make for a worse state of affairs (though it may be that the bad consequences of an act are precisely what led us to call it 'evil' to begin with!).
So I tentatively conclude that morality is means to a worthwhile end, and only a means. But if anyone can think of a counterexample which shows morality can add value independently of non-moral value, I'd be curious to hear it.