The little things add up (especially the negative little things), and, I mean in a big way. Research has shown our brains to be at least two times more biased towards remembering negative events and thoughts than positive ones.
Evolutionarily, this makes sense. If you didn't remember where the predators like to hide, or which plants were poisonous, then you didn't live to pass on your genes. If you simply forgot where the good berries grow, then you were just out one meal. Therefore, the evolving human brain learned to assign more value to negative events and information. Now take this into the modern brain and the modern world. We still have the same brain, except this time the negativ