I believe this quote means that fear is such a primal instinct, that even at the best of times, it can take over any good that we were feeling in a second. Fear often makes people react in ways that they normally wouldn't react, it makes people a shell of who they once were, ether it be for a brief moment, or for years of emotional turmoil. We never know how fear will affect a person, this is where the phrase "fight or flight" comes in.
