Loneliness can be different. Sometimes it is desirable and even necessary when we want everyone left alone. And then, it is a break from the intense contact with the World. So alone as the satisfaction of the need for privacy and taking care of yourself. Privacy for some time, and then again in the World.

loneliness as a transition or complete the process of parting or separation. The “ monad” when I learn of yourself and the World: I learn, I try, I can do what you are capable of and what yet, develop its resources, build plans for the future.

More loneliness as not being able to build contact with other...When there is a need to communicate the acceptance. When really want to share experiences when they want to be seen when you want to share your experiences and your World with Others. To be understood in my vision. See response in the Other what they say. When you have something to share, actually...And the possibility of it all. For reasons of fear of interacting with the world, fear of past experiences or lack of positive experience and the inability to rely on it, because there is no knowledge.

this is the avoidance of contacts or minimizing them in your life. There is no commitment to take the risk to manifest itself in your needs, do not have the security measures to get out of "comfort zone" where needs are met.

Fear that the other will reject, not choose. And then the loneliness - as a way of protection from rejection. There is no reliance on its value, another may choose you, just being you. The fear of losing one(mu), so the choice is to be made, not that I want to be with the Others, and that I want to be with someone, not only one(mu).

Often in the past negative experience where relationships with Others was dangerous. They present the elements of violence, disrespect, any close relationship are seen as not safe...therefore it is safer to stay without a relationship.
All this does not satisfy your need for communication, and we experience "pain" alone.