“The effect of small discords scattered throughout modern life can take their toll on the brain modules that regulate anxiety and depression. These modules are exercised by frequent use, meaning that the more stress, anxiety, and depression you experience, the more these negative states will become a salient part of your conscious life. Physically, activation of the hypothalamic-pituitary-adrenal axis, which causes stress, becomes more pronounced over time in people prone to experience stress. High activation of these negative modules leads to a situation where the natural positive mood of human beings may no longer be the default condition.”