Tell your class this… science suggests that our brains have an innate negativity bias. That is, we are wired to look for bad news. This is an evolutionary hangover – our ancestors needed to be hyper-vigilant in order to survive. In short, those hunter-gatherers who spotted the dangers quickly could avoid them (and therefore survive and pass on their genes) but those who stopped to smell a flower or admire a beautiful view were gobbled up by a lion. So, our brains are primed to look out for bad stuff more than they are tuned to notice the good stuff.
Then set children the task of looking out for situations where they or adults are noticing bad news, and introduce regular opportunities to celebrate good news. Like the examples here , from Adrian Bethune, founder of teachappy.