Negative thoughts have a way of inserting themselves unbidden into our minds. They reflect the unhappiness, perversity, and tragedy in our past and in the world about us. Only a Pollyanna would be ashamed to acknowledge them.
Negative thinking (rumination) does little harm as a long as it simply passes like a shadow across the otherwise sunny landscape of the mind. But negative thoughts bear a burden of emotion. They tend to plant themselves squarely in our path and grow roots. We dwell on them, sometimes for hours at a time. In certain cases this can lead to genuine depression. More often, habitual negative thinking tends to make people unhappy, pessimistic, cynical, suspicious, and morose. It also wastes precious resources of time and emotional energy. And it can lead to anxiety and depression.