I think that there are natural processes that produce human values. For one thing, I think that we as mammals evolved to care for our young. The emotion of love enabled us to delay self-gratification to care for the next generation. I think that evolution resulted in advances to make us feel emotional rewards that enabled us overcome the "selfish gene" in order to cooperate - not only to tend our young but also to have friends and kinship groups.
I also think that there are processes at work that generate communities of people who value such things as honesty. This is because a community that did not value honesty would become weakened eventually, and a stronger community that valued honesty would dominate or change it. Same goes for too much violence in a community. But then, there are conflicting processes that result in the opposite. If everybody in a community is honest, they all become trusting, then they become gullible and an outsider or a bad apple can wreak havok. So there will be some "selection" for some dishonesty to keep a community strong. Also a "selection" for some violence and aggression for a similar reason.
Of course, right now we're witnessing in our world many examples of the strong acting in ways that make them outcompete or outdo the rest of us in some way without ultimately benefiting the common good. Like predatory lending practices causing the housing bubble, for just one example.
And here is where I'll just lay out my own personal interest in ethical issues. I care about things like global warming, species extinction, peak oil, torture, food scarcity, and many other things. In my experience discussing ethics as a philosophical issue, I don't find that it goes in a practical direction. Which would be the direction of enabling people to make a better decision after thinking about it. As opposed to analysis paralysis. And this then leaves a whole arena of action to politics - without that politics being very guided by philosophy, except for whatever default worldview the players have. And the biggest players aren't playing on the basis of principles. They're not the dreaded ideologues. They just want wealth and power. And many other people walking around are also not ideologues - they just want to get their paycheck, shop, watch TV and drink beer. Everybody seems to have a fear that strong ideas will lead to ideology, but I feel they go overboard and disable ideas themselves, where ideas may be able to help our world. Which is a big part of my frustration with things like relativism. Somebody said in another thread that relativism permits no standard for criticism, and it shared that trait with dogma. I agree with that poster in that we need to avoid both relativism and dogma so that we don't disable thought itself as something that can help us make better decisions for our own lives and for the common good. And I keep using "common good", which would be instantly philosophized away in a debate, which is also kind of frustrating, because most people can agree that they don't like the way Hurricane Katrina was handled, for just one example.
Getting back to the nature of values, however, I think it's possible to explain, at least somewhat sketchily, the natural processes that result in people having values. But in another sense, how can you say it's good for communities to even survive in the first place? The people in the community think it's good (and I think it's good), but how can it be deemed good in a metaphysical sense? A demon on Buffy the Vampire Slayer would not care. Q on Star Trek would not care. A human being who was very philosophically inclined, who took a transpersonal perspective that "well, we're all just energy anyways, and we'll all return to energy, so living and dying don't really matter in the grand scheme of things, anyway" would also not necessarily care about what the people in the community think is good.
It's like you have to not only be a human being, but also be a human being who has a standpoint that is about living in the present as a human being, and valuing this life and not the afterlife, and many other criteria. I think by default we're like this but some people think their way out of it or use Eastern disciplines or even take acid to alter their perspective.