The psychological definition of loneliness hasn’t changed much since Fromm-Reichmann laid it out. “Real loneliness,” as she called it, is not what the philosopher Søren Kierkegaard characterized as the “shut-upness” and solitariness of the civilized. Nor is “real loneliness” the happy solitude of the productive artist or the passing irritation of being cooped up with the flu while all your friends go off on some adventure. It’s not being dissatisfied with your companion of the moment—your friend or lover or even spouse— unless you chronically find yourself in that situation, in which case you may in fact be a lonely person. Fromm-Reichmann even distinguished “real loneliness” from mourning, since the well-adjusted eventually get over that, and from depression, which may be a symptom of loneliness but is rarely the cause. Loneliness, she said—and this will surprise no one—is the want of intimacy.
Via The Learning Factor