Decent book, but a bit too simplistic and pop for my taste.
Basically the story is that nearly all of our mental problems - anxiety, depression, insomnia, etc - stem from an overly stimulated dopamine system, which in turn is caused by our modern overly stimulative environment. Gambling, smartphones, nicotine, porn, fatty foods, sugary foods, salty foods, all stimulate us to the point our brains increase dopamine levels and something called DeltaFosB. The first - dopamine - is meant to energize us for an incoming task, readying us for a reward or danger. The second - DeltaFosB - regulates the satisfaction we get from rewards. If both dopamine and DelfaFosB are too high for prolonged periods of time, we become fixated to rewards (addiction) or danger (anxiety) and DeltaFosB guarantees whatever satisfaction we derive will only make us require higher intensities while depriving us of satisfaction from weaker sources. This makes us jittery, anxious, irritable, addictive, insomniac, depressive etc. The way to fix this is to severely regulate our indulgence in stimulative substances and behaviors, bringing our dopamine and DeltaFosB back to baseline and with it our mental health.
Some other details: all addictions work the same - an addict will become extremely vulnerable to other addictions as their brain is primed for such; stress works also like addictions, priming us for dangers and therefore via dopamine and DeltaFosB also for rewards.
Our brain works this way in an attempt to adapt to an extremely rewarding or dangerous situation. But of course this was only ever a short term solution, for historically there has never been such a prolonged period producing chronically high dopamine and DeltaFosB, except for the modern society. Which is why we need actively be cognizant of our monkey brains and their monkey tendencies.
Also the author is skeptical of modern pharmaceuticals, which I find justified. Where I disagree with the author is in him completely disregarding psychology and character while attributing everything to those pesky brain chemicals. Afterall, all our thoughts are chemicals and so it achieves nothing to take the purely mechanical approach.