A quote from Ronald Rolheiser, The Restless Heart
To be human is to be lonely. To be human, however, is also to respond. The human person has always responded to this pain.
Sometimes it has moved us to greater depth of openness towards God and others, to fuller life, and sometimes it has led us to jump off bridges, to end life; sometimes it has given us a glimpse of heaven, sometimes it has given us a glimpse of hell, sometimes it has made the human spirit, sometimes it has broken it; always it has affected it. For loneliness is one of the deepest, most universal, and most profound experiences that we have.
Even if you are a relatively happy person who relates easily to others and who has many close friends, you are probably still lonely at times. If you are a very sensitive person, the type who feels things deeply, you are probably, to some degree, lonely all the time.