These two define a lot of how and what we see and are.
Their contrast is so much part of our life, it's can be as hard to grab as the air we breathe, absence of light would define death just as thoroughly.
Day and night, summer and winter, warmth and cold, growth and decay, being in a light or dark mood, are all bound to these conditions - and we need both.
When I worked in Iceland for some months, it was really hard to get to sleep when the sun was shining all through the night. A long and dark winter can drag you down into despair any your hunger for sun and spring becomes overwhelming.
And all our perception of colour depends on the wavelengths of light-rays, similar to how the wavelengths of sound define the tone we hear. If falling through a prism or other glass-object and separated, those colours become so pure, it almost hurts.
The bokeh effect you get when there is a lot of depth-of-field blur and very bright points of light can sometimes look like a lifeform in itself...
"The broken places are where the light comes in" says the Poet Rumi.
But we need the shadow too.