According to Rousseau, the human being is naturally good, and lived peacefully before the advent of civilization.
If it is the case, then why does Rousseau advocate for a social contract which purpose is to prevent the individuals from fighting each other?
This question is not about how the civilized society emerged, but about an apparent contradiction in Rousseau's thought: if humans are naturally good, why do we need to prevent them from being bad (via the power of the state)?
if humans are naturally good, why do we need to prevent them from being bad (via the power of the state)?
The premise is inaccurate insofar as it obviates Rousseau's distinction between savage man and civilized man. Rousseau certainly attributes to the savage man the attribute of natural compassion, and altogether characterizes him as "rather wild than wicked" (all quotes here are from his work "On the Origin of the Inequality of Mankind").
In addition to that state of wildness, Rousseau identifies in the savage man related attributes such as the rule of instincts, a lack of foreseeability, of dependence on other men, a lack of passions, and inextricably a lack of understanding. By contrast, Rousseau's term civilized man refers to man with passions, understanding, planning, a sense and sake of private property, ideals of self-preservation, and entangled with relations of dependence on other individuals.
Rousseau's premise is that civilized man's attributes "made man wicked while making him sociable". These also made him paranoid from realizing that other individuals are like him. This paranoia or fear leads civilized man "to pursue the rules of conduct [...] for his own security and advantage".
why, for instance, doesn't he advocate to return back to a pre-civilized state
Because Rousseau acknowledges that civilization is essential to progress. For instance, without society, "[e]very art would necessarily perish with its inventor, where there was no kind of education among men, and generations succeeded generations without the least advance". Rousseau asks "What progress could be made by mankind, while dispersed in the woods among other animals?", an obvious allusion to living in a state of nature.
From this standpoint, civilization is a necessary evil. But enacted laws supposedly are for curbing the "enormous injustices" that inevitably ensue from society.
Rousseau doesn't say humans, as savages, are good—he says they are not bad. From the Dissertation On the Origin and Foundation of The Inequality of Mankind :
Above all, let us not conclude, with Hobbes, that because man has no idea of goodness, he must be naturally wicked; that he is vicious because he does not know virtue. . . . Hobbes did not reflect that the same cause, which prevents a savage from making use of his reason, as our jurists hold, prevents him also from abusing his faculties, as Hobbes himself allows: so that it may be justly said that savages are not bad merely because they do not know what it is to be good: for it is neither the development of the understanding nor the restraint of law that hinders them from doing ill; but the peacefulness of their passions, and their ignorance of vice: tanto plus in illis proficit vitiorum ignoratio, quam in his cognitio virtutis. (Justin, Hist. ii, 2. So much more does the ignorance of vice profit the one sort than the knowledge of virtue the other.)
Also, quoting Derrida quoting Rousseau in Of Grammatology, pages 187-188:
They were not bound by an idea of common brotherhood and, having no rule but that of force, they believed themselves each other's enemies. . . . An individual isolated on the face of the earth, at the mercy of mankind, is bound to be a ferocious animal. [Essay on the Origin of Languages, pp. 31-32]
Rousseau does not say "they were each other's enemies" but "they believed themselves each other's enemies." It seems to be that we have the right to, and indeed should, consider that nuance. Primitive hostility comes out of a primitive illusion. This first opinion is due to a misguided belief, born of isolation, feebleness, dereliction. That it is only a simple opinion and already an illusion appears clearly in these three sentences that must not be overlooked:
. . . they believed themselves each other's enemies. This belief was due to their weakness and ignorance. Knowing nothing, they feared everything. They attacked in self-defense. An individual isolated on the face of the earth . . . [Essay, p. 32. Italics added.]
