Einstein's 'Theory of Relativity' is, at the end of the day, all about perspective. How you see things is a direct result of your position in space.
So what if we made the choice to put our headspace in a positive place? So that what we saw was through a filter pf gratitude and joy?
This doesn't always work. The glass half-full conundrum fails miserably when the glass is shattered by illness, depression, death, poverty, war or a myriad of other things life likes to throw at us. People out there are suffering well beyond anything considered humane. That's indisputable..
But I bet 95% of the time, for most of us, happiness is a choice. Those of us who are lucky enough to live in the middle, in relative comfort, should exercise that when we can.