T 2.3.4.5

This happens, among other cases, whenever any object excites contrary passions. For 'tis observable that an opposition of passions commonly causes a new emotion in the spirits, and produces more disorder, than the concurrence of any two affections of equal force. This new emotion is easily converted into the predominant passion, and encreases its violence, beyond the pitch it wou'd have arriv'd at had it met with no opposition. Hence we naturally desire what is forbid, and take a pleasure in performing actions, merely because they are unlawful. The notion of duty, when opposite to the passions, is seldom able to overcome them; and when it fails of that effect, is apt rather to encrease them, by producing an opposition in our motives and principles.