But I believe there is a still deeper and more common cause of loneliness. To put it very briefly, a person is most lonely when he has dropped something of his outer shell or facade---the face with which he has been meeting the world---and feels sure that no one can understand, accept, or care for the part of his inner self that lies revealed.
Each person learns, early in life, that he is more likely to be loved if he behaves in certain ways which are approved by his significant others than if his behavior is the spontaneous expression of his own feelings. So he begins to develop a shell of outer behaviors with which he relates to the external world. This shell may be relatively thin, a role he consciously plays, with at least a dim awareness that he, as a person, is quite different from his role. Or it may become a tough shell or armor plate, which he regards as himself, quite forgetting the person inside.
Now when the individual has dropped some of his defensive shell, is the time when he is most vulnerable to true loneliness. He may have dropped his facade, or a portion of it, voluntarily, in an attempt to face himself more honestly. Or his defenses may have been breached by an attack. In either case this leaves him with his inner, private self somewhat exposed---a self which is childish, full of feeling, with lacks as well as adequacies, and with both creative and destructive impulses---an imperfect and above all vulnerable self. He feels sure that no one could understand or accept the hidden self---an absolute certainty that no one could like or love this strange and contradictory self he has tried so hard to conceal. Hence there develops a deep sense of alienation from others, a feeling that "if anyone comes to know me as I really am, inside, he could not possibly respect or love me." Of this loneliness he is keenly aware.
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So loneliness exists at many levels and in many degrees, but it is sharpest and most poignant in the individual who has, for one reason or another, found himself standing, without some of his customary defenses, a vulnerable, frightened, lonely, but real self, sure of rejection in a judgmental world.
Rogers is writing about existential loneliness in "normal persons", participants in his encounter groups, but the experience he describes is deeply familiar to us, isn't it? I post this here to remind me, and you too, that we are not alone. We are not uniquely suffering when we discover the confinement of our dissociative facade and strive to become our more real, more complete self.