Why does O'Brien believe that everyone will break in Room 101?

O'Brien believes that everyone harbors a fear that will break them. Sometimes that fear is something is even non-life threatening. In Winston's case, his ultimate terror is rats. O'Brien says that it is human instinct to avoid this kind of terror. Given a chance to avoid the terror, a person will take it. He goes on to ask if a drowning person will grab a rope? O'Brien explains there are no heroes when it comes to a person's unique fear.