You are confusing the questions “why be moral” and “what is moral.”
Perhaps. I'm jumping back and forth a bit. I think these two things are related, but my initial question was about "what is moral" so I'll try to stick to that.
Morality is based on human flourishing, and self-interest is a reason to be moral. When you ask for moral justification for a given action, I’ll tell you in terms of the consequences for society at large, which in turn affects you as you are a part of society. This is both a moral justification and a proper motivator. I don’t know what else you need.
All right, let me try to lay out my thought process here, and you tell me where I've erred.
If what is moral is "that which is conducive to human flourishing" then that's fine. Self-interest as a motivator for such behavior is also reasonable enough; there are potential pitfalls, but then those exist with just about anything. That all seems fine.
My issue is that, unless there is some external standard of what constitutes "human flourishing," then you run into a problem. It doesn't have to be some "morality" that exists in the aether, but it would have to be
something. Two people can agree on what the consequences of a given thing will be on the world, but if they disagree on whether those consequences are actually conducive to human flourishing, how could a third party determine which person is correct? If a person opines that "X is conducive to human flourishing" (or in other words, that person states that X is right), and a second says otherwise, you would need to be able to look at those two statements and assess that one of those two is incorrect in their position, would you not?
So if a third party looks at this disagreement, they might be able to look at the consequences of X to make that determination (which is what I believe you may be saying). They could look at X and reason that X is clearly not conducive to flourishing, and that therefore someone who says "X is right is incorrect" in saying so. That begs a different question, though: how do you
know X is not conducive to human flourishing? The only way you could really do that is to have some definition or standard of what human flourishing is, so you could say that someone could be incorrect in their opinion of what is or isn't conducive to human flourishing.
That's what I mean when I talk about an external moral standard. I don't mean a rulebook floating around in the land of pure ideals.
If I'm misrepresenting you somewhere then tell me.