The psychiatric patient is alone. He has few relationships or perhaps no relationships at all. He lives in isolation. He feels lonely. He may dread an interview with another person. At times, a conversation with him is impossible. He is somewhat strange; sometimes he is enigmatic and he may, on rare occasions, be even unfathomable. The variations are endless, but the essence is always the same. The psychiatric patient stands apart from the rest of the world. This is why he has a world of his own: in his world, houses can sway forward, flowers can look dull and colorless. This is why he also has a special sort of body: his heart aches, his legs are weak and powerless. His past, too, is different. His rearing has failed, and this in turn causes his difficulties with other people - difficulties that summarize, as it were, all his other complaints. He is alone. He is a lonely man. Loneliness is the central core of his illness, no matter what his illness may be. Thus, loneliness is the nucleus of psychiatry. If loneliness did not exist, we could reasonably assume that psychiatric illnesses could not occur either, with the exception of the few disturbances caused by anatomical or physiological disorders of the brain. We have no knowledge of animals ever having "genuine" mental disorders.
The clearest articulation of the thought that loneliness is the dying heart of psychopathology. J H van den Berg; A Different Existence pp. 105-6.