Humans have emotional ups and downs because we've inherited the operating system of earlier mammals. The mammal brain releases dopamine when we approach a reward, and serotonin when we get respect. Oxytocin gives us a good feeling when we trust, and endorphin makes us feel good when we're injured. These happy chemicals evolved to do a job, not to surge all the time. Your brain defines that job in its own way because it built happy chemical pathways from your unique experience. You can build new pathways to turn them on in new ways. This book guides you through that challenge. It helps you choose new happy habits that are right for you, and repeat them until new circuits build. This is harder than you expect it to be. Our mammal brain evolved to rely on the circuits it built in youth, and to use unhappy chemicals as much as happy ones. But if you repeat a new behavior for 45 days, the electricity in your brain will start surging down the new path.