In Human Memory, Good Can Be Stronger Than Bad:

Some researchers assert that the psychological impact of negative information is more powerful than that of positive information. This assertion is qualified in the domain of human memory, in which (a) positive content is often favored (in the strength of memories for real stimuli or events and in false-memory generation) over negative content and (b) the affect prompted by memories of positive events is more temporally persistent than the affect prompted by memories of negative events. We suggest that both of these phenomena reflect the actions of self-motives (i.e., self-protection and self-enhancement), which instigate self-regulatory activity and self-relevant processes.