Why are you so upset that I did the wrong thing?

In Mere Christianity, C.S. Lewis begins by arguing for the existence of The Law of Human Nature. This is that humans have a universal sense that they should behave in a certain way (i.e., a morally correct manner) and they do not behave in that way.

In the second chapter he answers some objections. It took we a while to get my brain around the objections and the counter-arguments and so I attempt to summarise them here.

Objection 1: This personal sense of morality is just a herd instinct.
Counterarguments:
a. But the "herd instinct" to act for the greater good is in conflict with the "instinct" to act in a selfish manner and for self-preservation. But the sense of morality is what tells us to prefer one instinct over the other.
b. If only the two conflicting instincts are in our mind the stronger one (usually the selfish one) will almost always win. But our sense of morality leads us often to follow the weaker instinct.
c. Instincts are usually morally neutral. For example, the "fighting" instinct can both be used to protect the innocent and to murder. Our sense of morality tells us when its use is right or wrong.

Objection 2: The Moral Law is just a social convention.
Counterarguments:
a. Although, morality is "learnt" in a social context that does not preclude it being truth. Mathematics is also learnt and taught.
b. Although, there are differences in social conventions between different countries the differences in morality are not as great as argued. Futhermore, the fact that evaluate the relative merit of these different "social conventions" shows that we have sense of some absolute standard to compare with. Even those who would claim that morality is a social convention are unlikely to argue that slavery was not morally wrong.