Self preservation tactic. Happy, fluffy things aren't what'll kill you.
Because they tap into the caveman survival part of our brain.
Pessimism was selected-for when we were hunter-gatherers.
The trauma affects memory and stops the event from fading away.
I have no clue, but I do tend to remember the bad memories more so than good.
Adrenaline affects memory storage.
For me it is because I just happen to live in a corny horror movie type plot.
The uncommon things stick with us, like someone said below... The things that really impact us emotionally tend to have an adhesive affect. Unfortunately.
It is the same as asking "why do people remember pleasant, happy things" (unless you've never had any good experiences).
Because it's easier?