This piece by Hume sets out by remarking that all people are more or less equal until educated. In an early attempt to argue against the Social Contract idea, Hume remarks that we are all interested first and foremost in what we consent to about ourselves. He quickly reviews past and present governments and establishes that all of them are build on either usurpation or conquest. [While this might not outwardly be true anymore - most of our modern governments claim their rights to existence on democratic elections; their foundations still lie in what Hume deduced.]
To explain why this form of government is always prone to manifest, Hume offers that violence of false pretense will silence us to ask our fellow humans of their intention and thus of cunning person can make a following over many more than his numbers. His quick conclusion is that one born under a government (he mentions a prince), will be following this government because there is no actual choice to be had. He explains further that implying that a person without means to change his place (e.g. cannot emigrate) cannot be reasonably be said to have consented to any such contract, especially since this would imply that a vessel has also consented to the dominion of its master.
To prove his intent to not argue against the hypothesis of the consent to the social contract but his belief of its futility, Hume acknowledges that the most true tacit consent to the same lies with the foreigner who settles in another country while being informed about its laws and governance. Thoughtfully however, he adds that the allegiance to the new country, albeit voluntarily given (in contrast to the native subject), is neither expected nor depended upon.
Up until now, Hume has argued why consent is not actually present where it is said to be by the proponents of the social contract thesis, but now he goes on to explain by one must still adhere to the government and its laws regardless.
To have a stable society that is lawful and civilized, perpetuality has to be maintained. Humanity, he says, is in perpetual flux, the laws from before are carried to the after can only be tweaked but not abolished without destabilization.
So why should we be obedient to a government? Not because we gave our word - we have not consented but because only so can the necessities of life, which are served by a society intact, subsist.