Why does Cicero believe that the fear of punishment is not the only influence that leads people to be just or unjust?

Cicero argues that while the fear of punishment can deter wrongdoing, it is not the sole motivator for just behavior. He believes that true justice arises from a sense of moral duty and inner virtue, rather than merely the avoidance of consequences. For Cicero, the pursuit of the common good and adherence to natural law also inspire individuals to act justly, highlighting a deeper ethical commitment beyond fear. This perspective emphasizes the importance of character and reason in moral decision-making.