It was the high mark of Hobbes’ genius to elevate the concept of political obligation to serve as the cornerstone of his political theory: its main function, to spell out the necessary relationship between the subject and the sovereign. To drive in the final nail, Hobbes had to ensure the ensuing kind of obligation be a moral one rather than merely prudential, not therefore subject to an individual whim or a haphazard circumstance but forever binding. The foundations were laid.
To be sure, the idea of obligation, purportedly underlying most of our social structures, whether expressly political or not, is hardly surprising. In ancient Greece, for instance, the cradle of democracy and the birthplace of city-states, it was considered a solemn duty for every citizen to take active part in the life of the political community. In the Middle Ages, marked as they were by conspicuous absence of any type of political structure of note, let alone a durable one, political and social stability was maintained nonetheless by a complex set of reciprocal relations: serf to vassal, vassal to lord, lord to overlord. And underlying each of these relations there was the concept of personal obligation, a concept of duty; that’s what the Middle Ages were about. Indeed, even if we look further back, to our primitive or pre-political stage, to the life of a clan, a tribe, or an extended family, we’re still drawn to the very same conclusion: it’s our sense of obligation that made each of these possible.
In other words, the concepts of obligation and of duty were always present, albeit implicitly so, in all social structures with any degree of permanence, spelling out the conditions of said permanence; and they were implicit in the set of complex personal relations which in effect constituted those structures. It is arguable therefore that the subject’s sense of duty and obligation to a reigning monarch, let’s say, wasn't just to the institution of monarchy as such but also to the person or the personality of the king. Well, with Hobbes, sovereignty and personality undergo an irrevocable break. Hobbes’ idea of a sovereign, the rudiments of what was soon to become a modern state, is as impersonal as it gets. Hence the need to reposition these concepts front and center, to make explicit what previously was only implicit. Hobbes is merely following here the dictates of his grand project.