Rumination — not sadness — is what makes depression chronic

Research by Susan Nolen-Hoeksema at Yale found that people who respond to low mood by repetitively focusing on their symptoms and causes — 'why do I always feel this way?' — have significantly longer and more severe depressive episodes than those who distract themselves or take action. This process, called rumination, was more predictive of whether someone developed clinical depression than the severity of their initial mood. Nolen-Hoeksema's response styles theory has influenced Mindfulness-Based Cognitive Therapy (MBCT), now recommended by NICE for recurrent depression.

Source: Nolen-Hoeksema, Journal of Abnormal Psychology (1991)