The limbic system of the brain has been implicated as the seat of emotion, addiction, mood, and lots of other mental and emotional processes. It is the part of the brain that is phylogenetically very primitive. Many people call it "The Lizard Brain" because the limbic system is about all a lizard has for brain function. It is in charge of fight, flight, feeding, fear, freezing-up, and fornication.
In other words, the requisites for survival and continuity.