This is the star psychiatrist psychoanalyst of now. That is not to say that the book is bad, but it clearly needs to conform to some fads in order to be a best-seller. 

His main idea is clever and, ad hoc, obvious (generally, it's a hint that it is probably a good idea): emotions have a physiological basis, their repression also does. Not expressing emotions generates physiological stress (high levels of cortisol), and doing so chronically creates chronic stress and, in some cases, diseases.

He goes through several of them (ALS, MS, rheumatoid arthritis, fatigue syndrome, IBS, endometriosis, cancers...) and for each, unearths old epidemiological studies that also search for psychological and social variables: these diseases and their negative prognoses are correlated with bad life experiences, especially childhood experiences (specifically: ACEs, adverse childhood experiences), which shaped the way people react and deal with their emotions.

Said differently: if one does not know one's boundaries and let themselves be walked all over (because one was raised in a place where boundaries were not clear), the confused immune system can overreact in order to force the individual to draw proper boundaries (if you refuse to listen to anger or fear, your body might amp up the signal to make sure you listen). In other words: if you don't know who you are; your body doesn't either.