The idea of objective morality is one of the most appealing things about Orthodox Judaism. The concept of morality being subjective and therefore at least somewhat determined at the discretion of individuals is quite disturbing, especially as history has shown that people can create moral arguments to justify just about anything. Therefore, one could argue that for morality to exist at all in any meaningful form, it must be objective. Because only a truly objective source can determine objective morality, it logically follows that any objective moral system must be given by an omnipotent being, God. This God-given system would then be the objectively-correct way to live, and the moral value of any action would be determined based on its consistency with this moral code.
A typical response given to this claim is that the Torah endorses behavior that society considers immoral and therefore cannot be objectively moral. The fallacy of this response is that it assumes that the current societal definition of morality is in fact moral, and that therefore killing off nations, slavery, and permitting a father to marry of his minor daughter are in fact immoral. If there is such a thing as objective morality, and the Torah is the source of that morality, then those actions would in fact be moral and there would be no contradiction.
There is, however, a fundamental flaw with the objective morality argument. The argument assumes that if moral instructions are given by an objective source, the result will be objective morality. The difficulty with this argument is that the users of this moral code are fallible human beings. People do not generally think objectively and any intellectually-honest person will admit that his or her intellectual opinions are affected by emotions. Therefore we see that even those who claim to have a God-given moral code can and do modify the code as needed to satisfy their own, subjective, ideas of right and wrong.
The examples of this are too numerous to mention. Catholics believe in the unchanging divinity of the entire bible, yet today's Catholics would be horrified to see the church burn a heretic at the stake. How is it logical to believe that 500-years ago God supported the burning of heretics yet today he is opposed? There are no shortage of such instances within our Jewish tradition as well. Today rabbinic loopholes are found to allow things that are considered morally praiseworthy and prohibitions are created against otherwise-permissible things that are morally objectionable. Not only that, we routinely see people use Torah-sources to justify their personal moral opinions. Does the Torah support the prosecution of Jews accused of crimes? Well, it depends on who you ask.
Okay, so one may ask that if everyone makes subjective moral judgments, what harm is there in believing in objective morality? If a person believes in objective morality it may well lead one to attempt to consider opinions more carefully to try to determine if they are "objectively" right or wrong, which would seem to be a positive act.
The problem is that if one is convinced that there is a source of objective morality, one can therefore be more easily convinced by others that his moral instincts are incorrect as they oppose the objective moral source, or at least the speaker's interpretation of it. In its extreme form, this can lead an otherwise sane person to totally push aside his natural compassion and blow-up a building full of civilians. Even in more moderate forms, this line of thinking can lead people to discriminate against others, commit minor crimes,or oppose positive ideas.