Door Mr. Brian P. David
Even when we are on the "paths of righteousness," we are still under the threat of hell. And the weak point comes in the relationship between our internal, spiritual life and our external, natural life.
Valleys represent the lowest parts of ourselves: our day-to-day, bodily lives in the world. A shadow represents the clouded thinking that goes with that life, thinking connected to bodily things rather than spiritual ones. And death means spiritual death, or the obliteration of our desire to be good and our understanding of how to be good. So this verse is telling us where the threat is: No matter how exalted our thinking might get, it can get torn down pretty quickly by bodily desires. Well-intentioned diets are ruined by hunger; well-intentioned marriages are ruined by lust; well-intentioned businesses are ruined by greed. "The spirit indeed is willing, but the flesh is weak" (Matthew 26:41).
But we are protected by the Lord's "rod" and "staff." Those both represent the power of true ideas from the Lord; when used together, "staff" represents more external truth and "rod" represents more internal truth. So we have weapons! If we are indeed following the Lord, we will have powerful, applicable principles that can support us through any attack we suffer in the "valley."