Why does Rawls claim that his conjectural story about the situation in which justice is an issue involves no general theory of human motivations?

I do not want, therefore, to be interpreted as assuming a general theory of human motivation: when i suppose that the parties are mutually self-interested, and are not willing to have their (substantial) interests sacrificed to others, i am referring to their conduct and motives as they are taken for granted in cases where questions of justice ordinarily arise. justice is the virtue of practices where there are assumed to be competing interests and conflicting claims, and where it is supposed that persons will press their rights on each other. that persons are mutually self-interested in certain situations and for certain purposes is what gives rise to the question of justice in practices covering those circumstances.