Maybe we should start by asking, What kind of answer are you looking for? You might say, What an odd question. Surely, everyone fundamentally wants to be a good person and it would be odd to pursue the question further.
But we also need to think about the kind of individual who’s asking, Why should I be a good person? We might think that they’re asking what it is to be a good instance of the kind of thing they most fundamentally are – that is, what it is for them to be good, given what they are – namely, a person. From that perspective, it might seem odd not to want to be a good person. It’s a delicate question whether people ever aim fully for the bad as the bad, rather than by having mistaken what is in fact bad for the good.
You might be asking the question because you see people who are in some sense succeeding but don’t appear to be trying to be good. Many philosophers think that when one thinks about one’s ultimate end with respect to a course of action, that end is still conceived of as good, it’s just that one can be mistaken about what the good is. So one can pursue all kinds of ends, thinking they’re good in some sense, and just being mistaken about that. Maybe those people are striving to be good but failing, and the question for them isn’t, Why should I be a good person?, but rather, What would it take for me to be good?
“A Nazi might think being a good Nazi is part of what it means to be a good person – but be mistaken about that.”
And again, perhaps people can make mistakes about that. For example, a Nazi might think being a good Nazi is part of what it means to be a good person, but be mistaken about that. On that view, what people do is aim for the good, but miss, often missing badly. Such a Nazi might be a case in point. Maybe it’s just not true that these people aren’t trying to be good. Maybe they’re simply mistaken about what it takes to be good.
Another view of the question might be this: we have more or less primitive motivations, so sometimes our thoughts about the good thing to do will conflict with what I’m motivated to do by my more primitive urges. Often, when you ask, Why should I be a good person?, there’s a sense in which your question is really, How can I be a good person? The difficulty is living up to the demands of being good, given more primitive urges to be bad. In that case, it’s not obvious that one needs reasons to be good. Rather, one needs help in acting on those reasons.
In that case, it might be that one can know full well how to be good, but be unable to act in accord with what one knows; one’s conflicting motivations are making one unable to do the right thing. In that case, suppose you come to me and say, Why should I be a good person? One thing I could try to do is to help you govern your conflicting motivations, so that those motivations end up aligned with your affected motivation to be a good person. Now what I need to do is get your more primitive motivation to align with your knowledge of the good, in some sense appealing to your baser nature and get that to align with your less base nature. What does your base nature care about? “What’s in it for me?”
Plato is in some ways tried to engage with this form of the question in The Republic. He was up against discussants who thought that the fundamental goal in life is to profit from circumstances. There’s a sense in which that is everyone’s goal, to be able to profit from circumstance, and from that perspective the goal would be to argue or explain how being good is a way of profiting. How do I profit from being a good person? How will being a good person make me happy?
“Happiness isn't simply the feeling one gets of laughing hysterically in the park. It’s more of an ongoing flourishing in one’s life.”
Well, there's a tradition in philosophy of thinking of happiness in terms of flourishing. Happiness isn't simply the feeling one gets of laughing hysterically in the park, it’s more of an ongoing flourishing in one’s life, a way of one’s life going well.
Suppose, in that case, that it emerges that humans are, by nature, social creatures. It’s part of my nature to be a social animal, and part of being a good social animal is enjoying certain kinds of community with others, depending in turn on certain kinds of moral responsiveness to their needs. If that’s true then it would be part of my flourishing to be engaged in certain kinds of community with others, and just on that minimal basis, our flourishing will depend on what each of us does. So now there'll be certain demand that I’ll have to meet in order for others to flourish, and certain demand that others will need to meet in order for me to flourish. Crudely, to be a good person, you have to be nice to people. That helps make them happy, and helps make you happy.