Reddit mentions of Origins of Virtue (Penguin Press Science)

You have a moral gut feeling because humanity as a social species evolved moral instincts. Matt Ridley goes over this in The Origins of Virtue. These instincts however are not totally sufficient to guide us. After all those instincts were formed in a very different natural and social environment, and in any case will not necessarily always be correct just because they are natural.

Very few people think that ethics is 'objective' as in written into the cosmos or the mind of god. However many ethical philosophers have argued that Given the structure of mans mind(Immanuel Kant) or man's nature that we can argue that there is an objective way we should all act. Kant thinks going against his moral maxims is literally contradictory and therefore incoherent for example. Natural rights theories hold that the nature of man as separate, purposeful beings rationally implies that we should respect each other as such. Other moral theories posit an 'if you want this, then do this' kind of reasoning. For example do you wish the greatest happiness in the world? Then you must act according to utilitarianism. Hazlitt in his Foundations of Morality argues that your desire for your own and your loved ones well being and happiness implies that you should obey the moral rules that society has practically worked out for the social cooperation that can help you achieve it.

According to most moral theories the fact that societies accept different moral rules or even actively abhorrent ones does not let you off the hook in terms of a duty to be virtuous. It is not some cosmic force which compels you to be good but your own reason and conscience. Ethics is a practical thing. In the same way that medicine is a practical thing. We can learn to avoid bad things like slavery in the same way we learn that blood letting and frog swallowing don't cure cancer, and we can encourage good moral behavior like politeness just like we encourage good personal hygiene like hand washing.

edit: I am reading Arguments for Liberty (it's free in pdf and ebook) at the moment, which despite sounding like a political book is essentially about different moral theories, It can be a little challenging but it certainly helped open my mind to how the more 'objective' style moral theories can actually make sense. I can recommend at least the first three chapters (as that is where I am up to).

TL;DR

Your moral feelings are down to evolution. But that does not make them objectively right.
Moral rules are created by men but that does not mean that they are wrong or useless or that you should not obey them.