This is interesting:
Researchers report recalling specific positive memories from an earlier point in life was associated with fewer negative self-related thoughts and lower cortisol levels 12 months later. The study suggests recalling specific positive memories reduced vulnerability to depression over the course of one year.
The team found that recalling specific positive memories was associated with fewer negative self-related thoughts and with lower levels of cortisol 12 months later. In other words, remembering more specific positive events reduced their vulnerability to depression over the course of one year. Further investigation showed that recalling positive events only reduced negative self-related thoughts and depressive symptoms in response to stressful life events, but not if the adolescents had experienced no stressful life events.
More information: Recalling Happy Memories During Adolescence Can Reduce Depression Risk