The moment when a person's actual relationships fall short of desired relationships is commonly identified as the etiological moment of chronic loneliness, which can lead to physical and psychological effects like depression, worse recovery from illness and increased mortality. But, this etiology fails to explain the nature and severe impact of loneliness. Here, we use philosophical analysis and neuroscience to show that human beings develop and maintain our world-picture (our sense of what is true, important, and good) through joint attention and action, motivated by friendship, in the Aristotelian sense of "other selves" who share a sense of the true and the good, and desire the good for each other as much as for themselves. The true etiological event of loneliness is the moment one's world-picture becomes unshared. The pathogenesis is a resultant decay of our world-picture, with brain and behavior changes following as sequelae.