Why is it harder for some people to recover from stress-related illness than others?

Dr. David Clarke notes there is a very wide range of speeds with which people recover from stress-related illness.

One of his patients recovered in an hour. Another patient still had symptoms after going to therapy on a regular basis for 20 years. She wasn’t taking narcotics and she wasn’t being hospitalized anymore, but she was still having symptoms.

What accounts for such a wide spectrum of recovery time? Dr. Clarke says it has to do with the severity of the stress that someone is subject to and at what age the stresses occurred.