It is impossible to prove this conclusion as there may be a transcendental truth about morality which we should ultimately judge ourselves and other people by, but this transcendental truth is unknown to us. Of course, this stand also assumes the absence of a creator that establishes morals.
The first basis of morality was upon religion. Which I would argue is subjective. What was right to do in a society was based upon the society's religion. This didn't change until the age of reason came along and the idea of relative morality was actually considered an idea.
Morality implies judgement. Judgement of a group or an individual. This is highly subjective. Killing might be murder or war heroism depending on the standpoint. Thus Morality is created by humans. Without us the concept does not exist. Unlike physical laws that exist wheter we look or not.
Different individuals can hold different stances on morality. A Redneck and a civil rights activist hold very different moral stances on the treatment of minorities and each would describe the other as immoral while determining their belief to be moral.