We are naturally inclined towards the negative side of human existence. Over millions of years, the human brain has evolved into a structure that is inclined towards taking in the bad more than focusing on the good. So, evolution-wise, we have gradually become a species more focused on the negative. Our brains are constantly changing by slowly reshaping our neural structures. It is something called neuroplasticity. Therefore, what started as a behavior change towards the negative, neuroplasticity made it into a permanent change in the organic structure of the brain.