"Morality is subjective" is the inevitable conclusion of a secular, empiricalistic worldview.
Essentially, now that we are in a scientific world disagreement is resolved through experiment.
Disagreement not resolvable through experiment is removed from the realm of science, and is called falsifiable and is seen as subjective.
If you and I disagree, there are no scientific tests we can run to resolve moral issues.
And since we can't point to a God or objective moral laws, it doesn't even matter if one theoretically exists because it's inaccessible and infalsifiable. Effectively it doesn't exist for us.
Both of us are following different moral standards, the "rules" in your head are not the same rules that I'm subjective to.
You're morals are subjective to your experience, it simply is a fact.
Just human? I mean, sure do, but we're leaving out a huge array of animals who also engage in rudimentary moral behavior.
Of course, we evolved to be social animals did we not? What else would you expect but inate instinctual "rules" when they'd lead to a clearly fitter society.
Right, and just like the variation in genetic material this variation in inputs and outputs that we all have which are wholly unique to us as individuals and while remarkably similar to others raised in similar environments, also remarkably unique in subtle ways.
I agree this is the entire conversation. And the obviousness of this fact, that moral expression is subtly unique to each individual, is the ultimate answer to the question.
If you are raised in a subjectively different environment, then the rules you learn to behave by will be subjective to that environment.