Loneliness is a curious thing. Most of us can remember feeling most lonely when we were not in fact alone at all, but when we were surrounded (包围) by people. Everyone has experienced, at some time, that crazy sense of isolation(隔离) that comes over you when you are at a party or in an audience at lecture. It suddenly seems to you as if everybody knows everybody else; everybody is sure of himself; everybody, that is, except you. This feeling of loneliness which can make you weak when you are in a crowd is very difficult to get fid of(消除). People living alone are advised to deal with their loneliness by joining a club or a society, by going out and meeting people. Does this really help? There is no easy way out. Your first day at work, or at a new school or university, is a typical situation in which you are likely to feel lonely. You feel that everybody else is full of confidence and knows what to do, but you are alone and helpless. The fact of the matter is that, in order to live, we all put on a show of self-confidence to hide our uncertainties and doubts. In a big city it is particularly easy to get the feeling that everybody except you is leading a full, rich, busy life. Everybody is going somewhere, and you tend to believe that they are going somewhere nice and interesting, but your destination (目的地) is less exciting and attractive. What is the passage mainly about?