1. example or pressure from parents, professors, or any authority figure;
2. desire for fellowship with some non-religious person or social group;
3. desire to rebel against parents, professors, god(s), or any authority figure;
4. negative personal experiences with religious people or institutions;
5. distaste for the historical or distant actions of religious people or institutions;
6. distaste for the evils that are allowed by god(s);
7. personal injustice or victimhood;
8. personal misfortune such as disability, injury, illness, or the misfortune of a loved one;
9. personal failure or crisis related to substance abuse, gambling, guilty conscience, imprisonment, etc.;
10. personal dissatisfaction with one's social, romantic, or vocational circumstances;
11. desire to feel less guilty or behave more permissively; or
12. pride in my independence from the god(s) who might grant me eternal life.
Brian Holtz of the interesting site HumanKnowledge.net claims to have converted to atheism for none of these reasons.
Holtz's philosophy is "autocosmology." Its good attributes are epistemological empiricism, economic capitalism, constitutional federalism, and technological optimism. It wrongly asserts metaphysical naturalism, ontological materialism, and theological atheism.
One reason Holtz rejects Christianity is that Hell is supposedly unjust. Such an objection is unfounded. Jesus Christ, who was God the Son (stay tuned for future postings), taught the existence of Hell. The Bible teaches that there is a Hell. The Christian God exists and His justice, love, and sovereignty demand a Hell. Human dignity demands a Hell. The Cross of Christ implies Hell. Damnation for temporal sins is not overkill. God does try to reform people, but this happens during life on Earth. God cannot force free creatures to be reformed. Hell has redeeming value. 
Holtz's other arguments against theism and Christianity, as well as those by the Internet Infidels and StrongAtheism.net, have been thoroughly refuted; just search the web and do some reading; you don't have to take my word for it.  See also this funny parody  and this Fundy Top 10 List.  Read this interesting examination of atheist psychology. 
 Norman L. Geisler, Baker Encyclopedia of Christian Apologetics 310-315. (a must-buy!, 10/10)
Many other sites [please visit http://thebananarepublican.blogspot.com/2006/01/many-of-brian-holtzs-arguments-against.html]