When people disagree with me on matters of morality, they often ask, "why do you think that's a sin / moral duty?" This question shows a fundamental misunderstanding of morality. Why is it wrong to steal? This is a one-way conversation, so I'll have to guess at what your answer might be (feel free to contact me - I know I said I didn't want to argue but I'll make an exception for this). I'll make a sort of indented tree of possible responses.
Maybe you'd say, "It's wrong because it deprives other people of their property without their consent" (deontological answer). In that case, what's wrong with depriving others of their property without their consent? Who says that's evil?
Or maybe you'd say, "It's wrong because it's selfish/cruel" (virtue ethics answer). In that case, what's wrong with being selfish or cruel?
You might argue: "Being selfish or cruel is morally evil". But that's circular. I asked what's wrong with it, and 'morally evil' is a direct synonym for 'wrong' in this case. You were able to make the first jump because at least selfish is a word that can be given a clear definition and there's no way to deny that enriching yourself at the expense of others fits that definition. But you can't make the jump from selfish to evil because 'evil' doesn't have a non-normative definition, so you're forced to say "it's evil because I say so".
Or you might argue, "It's wrong because it causes suffering" (consequentialist answer). But what's wrong with causing suffering?
The point is that there is no why. Morality must necessarily have its roots in axiology, which exists outside of reason. It is the role of conscience to decide what things to consider morally valuable.
Actually, there's a slight complication: conscience doesn't tell you that. All conscience really is is your experience of either Fuiki or guilt after taking an action. From those experiences, you can derive a principle or rule that tells you what things are morally valuable (32 traits of behavior if you ask me), and that derived moral system can be used to predict what actions are moral without the direct use of conscience. Of course, I should say that it isn't strictly necessary to do this philosophy to be a good person. Your instinct inevitably learns by association to predict whether a given action will give you Fuiki or guilt. (This is the same way people often have a sense of how a practical science experiment will go without consciously knowing the underlying principle.) The only problem is that instinct can never explain itself, so if you want a moral system you can discuss with others and argue about, you need to philosophize.
A few more things I should say. First, reason is not useless in debates about morality. As I've implied, most people in such debates are basing their position not on an actual conscientious experience, but on a rule they've derived from it. That's why it's so easy to be wrong about morality despite that we all have an infallible conscience. But we know that conscience can never contradict itself, so if reason can point out a contradiction between two moral rules, it means that at least one of them must be false, or at least missing an exception clause or something. This forces a rational opponent to reconsider their principles and correct their extrapolated system.
People often claim that their conscience tells them something that I and maybe you know is not the actual teaching of conscience. This delusion is possible due to a further complication: most people's systems aren't even based on conscience in the first place. We were all taught certain rules as kids - don't hit people, don't steal, don't be mean, don't break the law - and had them drilled into our minds so hard in our most vulnerable years. (Note that all of those rules are at least missing two exception clauses.) From these, we developed a set of intellectual beliefs about what constitutes "good" behavior. Thus we associated feeling proud of ourselves (not to mention being rewarded) with a lot of the wrong actions, and so now when we use our instinct to predict the morality of an act, we ask it whether we're going to feel that way about it rather than whether we'll feel Fuiki, because we've corrupted the definition of "good" to refer to the wrong roots. And we call those wrong roots "conscience" even though it isn't. The English language's lack of a word for Fuiki must be a major contributor to this disaster. This way, even if you explain to someone the concept of Fuiki, they'll internalize it in terms of the morality they already believe in (you can't easily explain it without referring to "good" or "right"), and so they'll claim that immoral actions give them "Fuiki".