Who Decides What Is Right And What Is Wrong?

The writer, C.S. Lewis, points out that you've got to know what a straight line is before you know what a crooked one is.  But the “straight lines” of morality, of what is right and wrong, are fuzzy in the minds of a lot of people. Where do we get our idea of what is right and what is wrong and can something be “right for you, but wrong for me?”