As indicated in Figure 14

As indicated in Figure 14, nonlonely participants showed greater activity in the ventral striatum, one of the brain's 'reward centers,' when they saw a pleasant image of a person (a smiling farmer) than when they saw an equally pleasant picture of an object (a flower arrangement). For the nonlonely, a positive image of another human being obviously meant something special—it gave them a specific emotional boost....

Lonely participants, however, when they viewed positive images of people, did not register the same boost.  —Loneliness, Cacioppo & Patrick