According to a popular contemporary contractualist account of moral motivation, the most plausible explanation for why those who are concerned with morality take moral reasons seriously — why these reasons strike those who are moved by them with a particular inescapability — is that they stem from, and are grounded by, a desire to be able to justify one's actions to others on grounds they could not reasonably reject. My belief that an action is immoral, on this account, triggers this desire, this source of my moral motivation, and consequently I am moved to refrain from such an action by the fundamental desire that my actions be adequately justifiable. Furthermore, it is this desire to which contractualism appeals in its account of the wrong-making feature of certain actions: an action is wrong if it would violate a set of rules which no one could reasonably reject.
There seem, however, to be two general worries about this picture: one, that the range of its application is too extensive; the other, that the range of its application is too limited.