Here is Alexander Lowen’s explanation from Joy.
The difference between a purely physical pain and emotional pain is that the former is localized and affects a limited area of the body, while emotional pain — also in the body — is generalized. Headache is a pain localized in the head, a toothache is limited to the area of the tooth, and a pain in the neck affects only the neck. In contrast, the pain of loneliness is felt throughout the body. Emotional pain sterns from the body’s contraction in response to the loss or disruption of a loving connection. Such experiences can be heart-breaking, especially when they happen to a child and are connected with a sense of rejection and betrayal. Since the pain feels life-threatening to the child, survival demands that the experience, together with its pain and fear, be suppressed. Suppression is achieved by numbing the body through rigidification or by dissociating from the pain. Both procedures cut off feeling, leading to a sense of loneliness and emptiness.
These conditions become painful when an impulse to open up and reach out arises and is blocked by the fear of rejection. Since these impulses cannot be completely suppressed as long as one is alive — they are the essence of the living process — the individual is in a struggle with his own nature, that is, with his body and its feelings. Actually, the struggle is between the ego, with its defense against rejection and betrayal, and the body, with its imprisoned heart. The tension which this conflict sets up in the body is experienced as pain. Surrendering to one’s nature and allowing the impulse full and free expression immediately reduces the pain and results in the pleasurable feeling of fullness and freedom.
– Alexander Lowen, Joy (pg. 87-88)