fear of public speaking

[youtube:cHl20RjQZbw;[link:What Causes Fear?];http://www.youtube.com/watch?v=cHl20RjQZbw&feature=related] What causes fear? Why does fear seem to cause some people to react irrationally while others go on without too many issues? What causes fear to paralyze a person from living their lives? Some people will focus on the fear, which lead to more problems. What are these problems ? ...