I tell you what? I'll break it down even more and use a specific example. One that is considered pretty cut and dry -- murder.
Explain to me why murder is wrong? If you think that it is trivial or self-evident please indulge me. Why is murder wrong? And none of this, well it's against the law or because it violates the principle of harming others because that just begs the question why is it illegal or why is it wrong to harm others.
Often trying to explain what appears to be a simple concept becomes not so simple when it's broken down to its essence. If you can't do it, I understand. It's not as easy as people think. At least to those who have actually thought through the issue.
Murder is against the law by definition, as Murder is a legal term, its defined as "unlawful killing", like I mentioned earlier, people in the old west were made "outlaws" and as such it was impossible to "murder" them as they didnt have the protection of law.
Now, as for killing someone, its wrong because people decided its wrong, animals kill each other all the time, they dont think its wrong, death is just a part of life.
As I said before we switched to morality we were talking about rights, people dedided that people have a right to life, thats why we created laws to protect people, and formed civilised societies, go look at at Africa, you have people dispensing street justice and chopping people up in the street, everyone watching joins in, not one of those people believe they are doing anything wrong, now, where are their morals?