Mildly optimistic people are convinced they have some degree of control over their lives. Depressed or people with mental health problems tend to be better at realizing when they have no control over certain situations (such as life, maybe?). A study to test this psychological witty wiseness the sad people have was done with light bulbs. And you can read about it here:
My parent’s have always been outwardly pessimists around me since a young age. Although, I have lived with optimistic and awesome people, it’s tough for me to ignore the other pessimistic roommates. My brain filters out the good and worries about the bad and often gets jealous of the good too.