If there is a true, absolute morality, then we strive to discover it.
If this morality is a true metaphysical reality, a true ultimate good to the universe, then what would discovering it entail? Surely it would lead to a greater sense of well-being, for individuals and for communities, than what would have happened otherwise.
Imagine we traveled to another planet and found a thriving civilization. It is interesting to wonder if such a civilization could thrive if murder was not looked down upon. Perhaps there are some universals for any society to thrive.
If a morality did not contribute to an individual or community's well-being, then it would not be a metaphysical reality. It would be arbitrary.
I believe that we ultimately justify morality in our deepest feelings, our intuitions, but if we are to argue that these deep feelings correspond to some sort of objective, "external" morality, if we say that these feelings are objectively true in some way in addition to our deep feelings - like the rules of mathematics - then there must be a criteria.
Conversely, if there were a superpower dictating morality (a god, a majority, a government, a dictator) and this "morality" did not support a greater long-term sense of well-being, then this "absolute morality" would actually be "grounded" in the desires of a subjective entity. And the rules put forth would appear arbitrary to those seeking a meaningful life. In fact, they would probably seem immoral.
Is there a true metaphysical reality to be discovered and proven over time, or are there only arbitrary rules put forth at the whim of a subjective personality or circumstance?
In other words, if there is an objective morality, then how do we define it or know it or prove it? Ultimately it comes down to our deepest feelings, and to prove these feelings correct, there must be a criteria, and the grounding for that criteria goes back to even deeper feelings.