I responded to a post asking how/why one becomes an atheist. I thought I would post it here because this is a pretty good summary of why I believe what I believe.
I am an atheist because I believe physical evidence is and should be the final arbiter in all questions about reality. I'm not entirely sure how else you could define reality.
I am an atheist because I believe that doubt (not faith) is the path to truth. This is to some extent the core principle of science, and it has proven a very effective method for dispelling misconceptions.
I am an atheist because even the briefest investigation into brain injury or disease would demonstrate that there is no reason to believe that our memories, thoughts, emotions or personality ("soul") are separate from the physical nature of our brain.
I am an atheist because religion is arbitrary. Most people in the world follow the religion of their parents. Culture plays a significant role in which parts of a religion are followed (and which parts are ignored).
I am an atheist because I understand that every part of my body is composed of the same elements that everything else in the universe is made from - hydrogen, oxygen, carbon, phosphorus, iron, etc. There is nothing inherently "special" about life. Life is just self-replicating molecules, some of which have managed to replicate more than others because they are better suited to their environment.
I am an atheist because there has never been any repeatable experiment that in any way demonstrated supernatural or psychic powers. More to the point, everyone who has ever claimed to have such powers and has been subject to scientific inquiry has proven to either be mistaken or, more frequently, and outright fraud.
I am an atheist because I find all (or more) of the wonder, beauty and excitement in knowledge and learning that others seem to find in religion (with the added bonus of actual evidence). I feel sorry for those who can't or won't experience it - though I'm sure the feeling is mutual.
Ultimately, I think everyone needs to decide whether they are going to follow reality-based and physically-based evidence and knowledge, or mythologically-based knowledge. I can't fully understand why anyone would choose myth over evidence, as it strikes me as intellectually dishonest, but it happens with great frequency.
It is possible to form a world view that fills all of the gaps in our knowledge with "God did it," and requires that all evidence contrary to this be dismissed as "Satan did it" or "God is testing me." This world view has proven to be mistaken on countless occasions, and it seems to me to be of little value. I believe it only persists because it denies all reason that might be used to argue against it.