Fear, GM. We feel insecure, but mostly we want to stay alive. This is our greatest instinct. Indeed we do this so often that we don't even think about it. We look left and right when we cross the road, we jump or startle when we hear a noise, or feel uncomfortable in crowded areas. We hate the dark and cross over to the other side when we see someone or hear footsteps.
The nature of the Soul is Freedom, but we have lost it temporarily. We are on our way back and become scared of things that get in our way. Yet as with all Truth, there is a paradox. We want to stay on Earth. We are pulled by life's ephemeral trappings and enjoy the pleasure and the pain.
Ultimately, we fear darkness because we came from Light, and unconsciously or consciously, we sense this, and are like pilgrims journeying home. Much Peace to You, my Friend.
P.S. For the Saint it is a natural part of living, like you say, but only from the standpoint that he is strong inwardly. He is further ahead on the same road that you and I are travelling. Peace.