What did Kierkegaard have against Hegel?

Georg Wilhelm Friedrich Hegel was the dominant philosopher of the early nineteenth century. His system — an account of history, logic, consciousness, and reality as the unfolding of Absolute Spirit — claimed to encompass everything. It was systematic, total, and abstract.

Kierkegaard thought this was precisely the problem. A system can contain everything except the person who lives in it. "The system" — any system — cannot capture the actual experience of an individual human being who must choose, suffer, believe, and die. To subsume existence into abstract categories is to lose the most important thing: the particular, irreducibly individual person.

His objection was not that Hegel was wrong about facts. It was that the project of systematic philosophy — of explaining reality from the outside, comprehensively — left out subjectivity, the only place where truth actually matters.

What did he mean by "subjective truth"?

Not that truth is a matter of opinion. He meant that the truth that matters most to a human being is not objective fact (which you can know without it affecting you) but truth that you have appropriated — that has changed how you exist.

"Truth is subjectivity" does not mean "whatever I believe is true." It means "the most important truths are the ones that have been personally metabolized, not merely known abstractly."

Why does this matter outside of philosophy?

Because we often treat knowledge as a substitute for transformation. We learn about meditation, exercise, diet, or relationships and believe knowing is enough. Kierkegaard would say: no. The knowledge has not become yours until it has changed how you live. That process — inward, difficult, irreducible — cannot be outsourced to a system.