Morality/ethics, defined as the construction of principles which increase individual and collective happiness and decrease suffering, is an objective enterprise. It is objective because certain courses of action are objectively better at reducing suffering and increasing happiness than others, in spite of the fact that morality varies among cultures.
Defining morality/ethics in this way captures the existence of culturally-transcendent intuitions of right and wrong which are fundamental to human beings as social animals. (For an example of a culturally-transcendent moral intuition, wiki the 'Trolley problem'.) Such a definition also allows us to take a more "scientific" approach to ethics (quotes emphasized): it allows rational, informed, sane scholars to come to a consensus on general principles (e.g. human rights are good, cocaine is bad, the Golden Rule is good) which increase individual/collective happiness, while allowing for exceptions to be made in special circumstances and alterations as knowledge increases.
I would add that such a definition does not deny that human beings have a fundamental dark side as well. While most of us agree stealing is wrong, if we were starving, most of us would give in to the temptation to steal, cheat, or murder in order to feed ourselves. Part of the enterprise of ethics, then, is to provide people with the basic necessities of life so they are not put in these impossible situations, and to rationally set up legal and social institutions which make it disadvantageous to be immoral and advantageous to be moral. There is also probably a part of us all that would derive happiness from inflicting harm on those we perceive as the "other" (the other tribe, the other race, etc.) This is an artifact of our evolutionary heritage, and it is not difficult to see why we will be happier if we restrict this impulse (e.g. through education and culture).
Of course, this is one person's opinion. There is a rich and voluminous tradition of philosophy, from Aristotle to Hume to Kant, which addresses ethics. On the other hand, appeals to divine authority are little help to us, particularly when they constrain our moral reasoning to one particular, unalterable strand of Bronze-Age myth.
Such closed-mindedness leads to what I believe to be indefensible moral claims: that, at one time, it was immoral NOT to collectively throw stones at adulterers or gays until they are dead; that everyone--even selfless, compassionate, good people--who disagrees that certain historical events (e.g. the Resurrection) occurred will be cast into eternal fire, and that this constitutes the ultimate expression of "love"; that we are forgiven for our misdeeds by a human sacrifice which occurred thousands of years ago in an obscure part of the world, rather than by admitting our wrongs and seeking forgiveness from the people whom we have wronged in this life.
Such closed-mindedness is further, in my opinion, the only way to explain the immoral (but tragically well-intentioned) behavior of the pious men and women who stoned adulterers and burned witches; or those who today are more concerned with the well-being of a blastocyst (a cluster of 100 cells from which embryonic stem cells are derived) than the well-being of every burn victim or child in need of an organ transplant; or those who were outraged when the lightning rod was invented, or who today deny their children a blood transfusion or encourage AIDS - riddled Africa not to use condoms, for fear of interfering with what they imagine to be God's plan.