There are two ways in which an individual can experience loneliness.
The first is the loneliness of being in the world. This is the common understanding of loneliness in which one feels his or her being alone in the world with others. In the loneliness of being-in-the-world, the individual feels no connection or relation with others, despite their ubiquitous presence. It’s the feeling one gets when one is amid a large crowd of others, but nevertheless is overcome with a sense of isolation and solitude.
The more profound form of loneliness is wholly internal, and characterized not by the absence of relations with other beings or entities, but rather the insight into the emptiness within. This is the loneliness of the world. One becomes lonely when one senses or feels the emptiness of the world but within his very being. It’s as though the groundlessness of the world is transcribed within the individual self, and thus the self too is equally empty and without any grounding whatsoever. This form of loneliness is much rarer than the loneliness of being in the world, and typically accompanies feelings of existential anxiety or dread, in which one’s being discloses the un-relatedness of existence on account of one’s (subjectively) coming to terms with one’s own death.
The feeling of profound loneliness can only be compared to an irretrievable loss; a delivery over into total and complete existential solipsism, where one transcends the vulgar crassness of the inauthentic relatedness of the crowd, but at the same time forever loses the capacity to take comfort in or derive meaning from his being-involved-with.