We react against the possibility of loneliness. 

Fear is a universal experience. Even the smallest insect feels it. We wade in the tidal pools and put our finger near the soft, open bodies of sea anemones and they close up. Everything spontaneously does that. It’s not a terrible thing that we feel fear when faced with the unknown. It is part of being alive, something we all share. We react against the possibility of loneliness, of death, of not having anything to hold on to. Fear is a natural reaction to moving closer to the truth.
If we commit ourselves to staying right where we are, then our experience becomes very vivid. Things become very clear when there is nowhere to escape.

-Brainpickings