Andrew Zak Williams has collated a fairly useful set of reasons/opinions as to why people do and don’t believe.
http://www.newstatesman.com/religion/2011/04/god-believe-faith-world-belief
http://www.newstatesman.com/religion/2011/07/god-evidence-believe-world
I planned on tucking into a few of the logical fallacies and desperate appeals to ignorance, but then commenter JP said of the first list above: “A nice long list of bad arguments, emotional appeals, illogical beliefs, and just plain wrong information about history.” So, I’ll defer to his wise statement and save myself the Repetitive Strain Injury.
Have a read though, if only to gain ammunition for your next debate …