We're all born with a natural survival instinct. Psychologists refer to it as a "negativity bias".
People that can sense danger have good memories for bad events. By remembering the pain they avoid future pain. They know what to avoid.
That's why people pay more attention to negative events than positive events. We look for danger as a means of survival.
Bad memories are more powerful and memorable than good memories. That's why we tend to overreact.
- Cacioppo JT, Cacioppo S, Gollan JK. The negativity bias: Conceptualization, quantification, and individual differences. Behavioral and Brain Sciences. 2014;37(3):309-310.