is the crucial idea denied by libertarian political theory. We can have obligations that we did not agree to take upon ourselves.
Here is David Walsh on that fact:
"The political is never merely an option, for we are embedded in a network of obligations before we even begin. This was the weak point of all social contract explanations of civil society, with their inevitable implication of the arbitrariness of a state founded on individual choice. Kant reminds us of the extent to which the state provides the conditions for the exercise of free choice and is thus beyond the realm of choice. We are obliged to support the political constitution under whose order we exist, not because we derive benefits from it or because we have given our consent, but because it is part of the order of being." -- The Modern Philosophical Revolution, p. 62.