In fact, it is not difficult to understand that such a shift occurs, and for relatively slight reasons. Suppose a stimulus X corresponding to the mood a is exerted upon a mass of people who are present in the same place. In this mass there is a number of individuals, perhaps one only, whose temperament and natural passion tend toward a. This individual is vividly stimulated by X, which reinforces his own leanings. Understandably enough, this person takes leadership in the mass, which is in some measure already disposed toward a, and which follows the mood of the leader whose temperament exaggerates the stimulus.

Georg Simmel, The Sociology of Georg Simm…, loc. 4161