I think the author's purpose in writing the book is summed up well on the back jacket of the book
This book is "practical advice to explain how prolonged stress causes or intensifies a range of physical and mental afflictions, including depression, ulcers. colitis, heart disease and more."
He is very interested in showing parallels and differences between humans, other mammals especially primates and laboratory rats.
"When we worry or experience stress, our body turns on the same physiological responses that an animal's body does, but we usually don'y turnj off the stress-response in the same way-through fighting, fleeing or other actions. Over time this chronic activation of the stress response can make is literally sick.
He is also bringing science to the non-scientist/layman. He is also telling us that are stressors can be managed because the really aren't as dire as the direct threat a zebra experiences. The final paragraph of the final chapter of the book I think gets to the meat of what Sapolsky is demonstrating in the book.
"Perhaps I’m beginning to sound like your grandmother, advising you to be happy and not to worry so much. This advice may sound platitudinous, trivial, or both. But change the way even a rat perceives its world, and you dramatically alter the likelihood of its getting a disease. These ideas are no mere truisms. They are powerful, potentially liberating forces to be harnessed. As a physiologist who has studied stress for many years, I clearly see that the physiology of the system is often no more decisive than the psychology. We return to the catalogue at the beginning of the firstchapter, the things we all find stressful— traffic jams, money worries, overwork, the anxieties of relationships. Few of them are “real” in the sense that that zebra or that lion would understand. In our privileged lives, we are uniquely smart enough to have invented these stressors and uniquely foolish enough to have let them, too often, dominate our lives. Surely we have the potential to be uniquely wise enough to banish their stressful hold."