Selected excerpts from the Wikipedia entry on "loneliness".
Loneliness is a feeling where people experience a powerful surge of emptiness and solitude. Loneliness is more than the feeling of wanting company or wanting to do something with another person. Someone lonely may find it hard to form human contact.
In their growth as individuals, humans start a separation process at birth, which continues with growing independence towards adulthood. As such, feeling alone can be a healthy emotion and, indeed, choosing to be alone for a period of solitude can be enriching. To experience loneliness, however, can be to feel overwhelmed by an unbearable feeling of separateness at a profound level. This can manifest in feelings of abandonment, rejection, depression, insecurity, anxiety, hopelessness, unworthiness, meaninglessness, and resentment. If these feelings are prolonged they may become debilitating and prevent the affected individual from developing healthy relationships and lifestyles. If the individual is convinced he or she is unlovable, this will increase the experience of suffering and the likelihood of avoiding social contact. Low self-esteem will often trigger the social disconnection which can lead to loneliness.
People can experience loneliness for many reasons, and many life events are associated with it. The lack of friendship relations during childhood and adolescence, or the physical absence of meaningful people around a person are causes for loneliness, depression, and involuntary celibacy.
Loneliness can evoke feelings that 'everyone else' has friends, or that one is socially inadequate and socially unskilled. A lonely person may become convinced there is something wrong with him or her, or that no one understands his or her situation. Such a person may feel reluctant to attempt to change or to try new things. In extreme cases, a person may feel a sense of emptiness, which may become a state of clinical depression.