Why can't humanity be more positive?

Society has this sad “mood”. Anywhere you go, it’s the same. Why can’t people be positive, and think about what is precious in their lives rather than focusing on the stressful things that’s going on? If they think positively, that stressful thing will change to just something bright!
I just don’t get why. Why is it so hard to smile at someone you randomly see? Why is it hard to communicate with workers in a store rather than being moody? Why is it so hard to reject making each other happy?