In “Dopamine Nation: Finding Balance in the Age of Indulgence,” Dr. Anna Lembke says today’s superabundance of pleasurable stimuli makes us all vulnerable to overindulgence. But don’t lose hope. Anna, the medical director of addiction medicine at Stanford, says that by understanding how modern stimulants — from Instagram to masturbation machines — prey on our primitive brains, we can find ways to overcome the unhealthy dependencies that prevent us from leading balanced lives.