In Ethics, subjectivism means you make the rules based on “I want what I want; who cares what everybody else wants?“. It is voluntarism.
The golden rule is better because it respects (recognizes and acknowledges in one’s thoughts, values, and behaviors) the personhood of every subject (person). This means we gotta care what everybody else wants (who ALSO care about what everybody else wants) when we think about what we want.
The only worldview that believes there is someone(s) described by the golden rule [meaning the rule is not only prescriptive, but descriptive/true… IF that someone(s) exists/behaves as original personhood/being] … is Christianity.