Tough Questions Answered explains how that question is answered on theism and on atheism. (H/T J. Warner Wallace tweet)
If you are a dictator, and you have complete control over your nation, and you have good reason to believe you will remain in control, why should you not take whatever you want from whomever you want in order to bring yourself pleasure? Why would it be rational for you to be moral?
In certain cases of truth telling or repaying a debt or keeping a promise, and in those rarer cases where the performance of a duty risks death or injury, why do the moral thing? In an atheistic world, there may be instances where doing the moral thing does not advance my goals and desires for my life. In other words, doing the moral thing may not be the rational thing for me to do.
TQA then quotes from “Good God: The The Theistic Foundations of Morality” by David Baggett and Jerry Walls:
It wouldn’t make sense if the world required us to do what isn’t in our ultimate self-interest. We think this was Kant’s insight when he suggested that the moral enterprise needs, in a deep and radical way, the postulate of a God who can, and will, make happiness correspond to virtue. Morality fails to make sense when that correspondence fails.
[...]It’s the atheistic world in particular, however, that introduces the failure of this correspondence. Reality itself must be committed to morality in some deep way for morality to make sense. Morality really must be a very deep feature and fixture of reality in order for its demands to retain their authoritative force. In an atheistic world there just doesn’t seem likely to be the sort of ontological foundation to morality that renders it always rational to both believe in and do what’s morally binding. The picture is very different for a theistic world of a certain sort.
One of the most obvious questions to ask atheists these days is whether it is OK to kill a special needs child. Atheists like Richard Dawkins and Peter Singer are already on record as saying that it is a good thing. It will be tough for an atheist to take any other position, because that is rational within their atheistic worldview. This is a clear case where the life of the strong person will be made less pleasurable if the stronger adult has to deal with the needs of the weaker child. On atheism, there is no such thing as a human right. The universe is an accident. Therefore, atheists will find it rational to kill anything and anyone who diminishes their happiness, so long as they can escape the consequences of being judged by others when they do.
On theism, though, it is rational to pursue morality, because the universe was made by a moral Creator, and his moral values are built into the design for us. Working to comply with that design is done within the context of that relationship with the Creator. A relationship which survives our deaths. Christian theists have even more reason to prefer self-sacrificial love, since the example of the life of the founder of Christianity exemplifies that self-sacrificial love. Caring for others even when it doesn’t make you happy is the core of Christian doctrine and theology, and that’s in fact what the church has done since the beginning.