Why do people become fanatics?

Both believers and atheist sometimes get to extremes about their opinions, and everyone who think different - is stupid / will burn in hell. Why do people get to that? How can they become so sure that there's no way they could be wrong, in this world where nothing in fact is sure (except for death and taxes) ?
Update: Deke - the lady in your story is a fanatic as well :) by the way I love Harry Potter
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