Fog

The valley is so dark.  So scary.  But the shepherd knows that the route to the high grounds is most safely travelled by way of the valley.  There are plenty of water sources in the valley to quench our thirst. 

The valley gets even more daunting when the fog rolls in.  At least in the dark we can make out shapes.  We can see certain things.  When the fog comes, though, everything is distorted.  We can't make out shapes anymore.  Our depth perseption is riddled.  We were staying close to the shepherd in the dark, now it is vital to our survival that we walk as close as we can to him.  He knows the way.