I think because life is harrowing, harsh, and daunting. Life is isn't easy, there is a lot of turmoil, sadness, loss of hope, crime, etc. As well, we're constantly dealing with adversity, decisions and things beyond our control.
That being said, there are also a lot of amazing things about life. Our families, the weather and nature, our job, living in a free country (for those of us lucky enough), kindness from a stranger, laughter of a child, etc.
Although a lot of bad things are going on in the world, there is also a lot of good things, and kind acts being performed, we just have to look for it.
I think sometimes people get so bogged down with the bad, that they forget about the good things in life. Which just drags them down more, making everything seem more challenging than it actually is.