Cicero's version of duty is not exactly the same as obligation in the same ways that one is obligated to pay one's taxes. That isn't quite what Cicero means by duty because if one fails to pay their taxes, they go to jail. It isn't voluntary. So, Cicero's version of duty is a response not to the existence of obligation, but to the obligatory nature of something else: human volition.
By focusing on themes like honor, family, and justice, the great thinker paints a portrait of life that has duty as a main motivator. Duty ends up becoming a full-blown metaphysical concept, and Cicero touches on themes that verge on Karma, alleging that if one follows their true fate, to execute their duty for the betterment of the community and family, then, the universe seems to align itself better with that person.
He doesn't say it is easy, and what's even more frustrating potentially, at least for some readers, is that Cicero starts with discipline, and one discipline in particular: Empathizing with others. He uses reason as the tool for justice, and he uses that empathy as a guiding principle. Cicero was expected to go into politics, but instead, he serves a different duty: Observing time with stoic focus, and performing correct action according to the ethical principles which he details.
