Generally speaking, psychosomatic illnesses are caused (or aggravated) by a mental factor such as internal conflict or stress because of the interaction of mind and body.
A psychiatric social worker told me the story of
working with a client who had a psychosomatic illness. Once it was
determined that it was a psychosomatic cause, they explained in specific
detail how the mind and body work together. Once the client recognized
and accepted the validity of what he was being told, his "illness"
instantly disappeared. This encouraged him to accept that the root
causes were emotional and thus became willing to pursue the counseling
that he'd previously rejected. The followup psychological counseling was
The key was accepting that the mind is distinct from the body. There's an interplay -- but they're not one and the same.