There are many metaphors about light and shadow, but I think the most important one is the idea that for better or worse, both are necessary in life.
Without shadow, you actually wouldn’t be able to perceive things in three dimensional space. Ask any visual artist and you’ll realize that shadow is what makes things stand out and look real. But too much shadow is a bad thing, as it obscures and hides.
Without light, we wouldn’t be able to see anything since it would all be darkness, and light is necessary to cast a shadow in the first place. With too much light we also can’t see anything as it all gets washed out in the brightness of an overpowering light.
What you want to keep in mind is that the aperture that controls the amount of light works the same as your perspective on life. Extremes aren’t helpful, but it’s in the nuance that you’ll find the best lighting for your individual situation.