Light and dark.  We deal with light and dark, physically as well as metaphorically, every day.  Oddly, we cannot have dark without light, for dark is no more than the absence of light, but unless we have experienced darkness, we cannot fully enjoy and appreciate light.
     Many centuries ago, the religion of Zoroastrianism emerged in the sands and mountains of ancient Persia, proclaiming a world dominated by a god of light (Ahura Mazda) and a demon, maybe more a demi-god, Ahriman, purveyer of darkness, who are constantly battling for supremacy of the universe.  Zoroastrianism understood that life consists of darkness as well as light, and that being human is to deal with both.
     As do we.  We usually do whatever we can do avoid darkness, yet we know in our hearts that we cannot elude it altogether.  And we of course embrace the light in our lives, reveling in the joy and wonder it brings us, hoping, perhaps, that it will never end.
     But it will.  As Zoroastrianism pointed out, however, light, embodied in Ahura Mazda, is, in the end, always stronger than the darkness of its counterpart.  It understood that, in the biggest of all pictures, light, driven by divine forces beyond imagination, will prevail.
     Why else would we live?