A question which I had not before considered, thank you for asking, Brian!
I agree with Lorna (great to see you on here Lorna!) and would add the following guesswork:
BODY
Have you ever gone a long time without really being touched by another and then one moment someone does something as seemingly insignificant as placing their hand on your shoulder? Or, better yet, gives you a hug? (Prior to my long term relationship with my girlfriend, I had long stretches of non-physical-contact with others. It actually need not be "long" by most standards. A period of a few days is enough.) Immediately you can feel the stress dissipate from your body, like extra heat lifting from your skin, leaving a cool sensation of relief in its wake.
The stress that is released is, itself, the loneliness being dissipated, I believe. The body has natural built-in desires to connect with other bodies. When there is no contact, there is a sort of stress, pain, or loneliness that is perceived in many but not all entities.
I would imagine that this loneliness is, as Ra said, a "natural function of the body" because the loneliness serves a helpful role in spiritual evolution, that is it begs an answer or solution. It predisposes entities to seek closer contact for the benefit, ultimately, of learning.
Session 15 Quote:The understanding, experiencing, accepting, and merging of self with self and other-self, and finally with the Creator, is the path to the heart of self.
MENTAL/EMOTIONAL
This type of loneliness, I believe, is as Lorna described it: a situation of not having another with whom to share mental/emotional energies, though the other-self as "partner" being only a special but important case.
This type of loneliness seeks its end in the connection with others with whom one can transfer mental/emotional energy. This happens, I believe, in the teacher/student relationship, the intimate partnership, or the joyful connecting of like minds, be they similar in their love for physics, pig latin, or Harley motorcycles.
Not to exclude the emotional component of this type of loneliness, perhaps there is a built-in loneliness which seeks company in the sharing of purely emotional experiences. One could make a case that two die-hard sports fans - especially the type that paint their entire bodies in the color of the team they support - are ending or at least ameliorating emotional loneliness as they share their total passion for their team, and the tears or the triumph that results.
SPIRIT
??
Perhaps, and this is only a guess, loneliness of spirit is a state wherein the self begins to feel acutely its perceived separation from the One. In such a case, this type of loneliness would not easily be placated by means of sharing energies with others. Rather, it is the primary connection between the entity and the One that is being sought.
This type of loneliness would in all probability, if understood to be the absence of the Creator in the entity's life, motivate the entity to begin a lifetime process of ardently seeking the truth... or, if the loneliness was misconceived, it might lead the entity into darker corridors of chemical sedation and escape.
At any rate, food for thought.
: ) GLB