Loneliness is the region of fear, which is rooted in the exposure of a being that must exist but is pushed out into a situation with which it is impossible for him to deal.
A concrete example may help to make this clearer. When a child has to walk through the woods in the dark, he feels frightened, however convincingly he has been shown that there is no reason at all to be frightened. As soon as he is alone in the darkness, and thus has the experience of utter loneliness, fear arises, the fear peculiar to man, which is not fear of anything in particular but fear in itself.
Fear of a particular thing is basically harmless; it can be removed by taking away the thing concerned. For example, if someone is afraid of a vicious dog, the matter can be swiftly settled by putting the dog on a chain. Here we come up against something much deeper, namely, the fact that where man falls into extreme loneliness he is not afraid of anything definite that could be explained away; on the contrary, he experiences the fear of loneliness, the uneasiness and vulnerability of his own nature, something that cannot be overcome by rational means.
The child will lose his fear the moment there is a hand to take him and lead him and a voice to talk to him; at the moment therefore at which he experiences the fellowship of a loving human being. The fear peculiar to man cannot be overcome by reason but only by the presence of someone who loves him.
— Joseph Ratzinger, Introduction to Christianity, II.II.3