Here's the thing about atheism. It is not an alternative to religion, or an entirely different mindset, and, above all else, is not faith based. Atheism is a skeptical approach to the claim of a god (actually, to the claim of any and all gods).
Mikey asked an interesting question: why atheism? While I can not speak for every atheist, I can tell you my reason for atheism. My reason is a statement of fact, plain and simple. Many will argue that this fact is incorrect, but I can tell you now that it is not incorrect. Why am I an atheist: there is absolutely no independently verifiable objective evidence that supports the claim for the existence of any god worshiped....past and present.
I am unable to understand why the vast majority of human beings insist in believing in the affirmative when it comes to religion, yet will take positions of skepticism when broached with nearly every other supernatural claim. Why is the default position to believe when told about god? Why isn't skepticism the default position. Skeptical approaches to claims is how we come to understand what is actually true and what isn't. Here's a fun little example. In 1989, a chemist by the name of Martin Fleischmann reported that he had achieved cold fusion in his laboratory. Fusion, combining the nuclei of two elements into heavier ones, is a source of great energy (it's what powers the sun). This energy, however, comes at great expense, since the temperatures needed to achieve fusion are incredibly high. However, cold fusion stated that fusion could be achieved at close to room temperature. When the claim of cold fusion was made, did everyone immediately believe it to be true? You already know the answer to that question, and instead, scientists ran back to their own laboratories to try to recreate the conditions which would allow for cold fusion. Well, turns out, cold fusion was all but a farce, and what was reported to be cold fusion was, in fact, not. Cold fusion is considered by most a dead topic, but some research continues to be done with little positive results. The point I am trying to make is that nearly every claim made must be supported by evidence before it is accepted as fact. What makes the existence of god so unique that accepting it as fact does not warrant our instinctual skeptical scrutiny?
One such reason may exist within the nature of the claim itself. Cold fusion was an okay example, but it differed from the claim of god in one single respect: it was falsifiable. Experiments could be done by independent parties to produce objective data that showed cold fusion to be false. There is no experiment or argument that can objectively show their to be no god. I can not devise a test that would produce objective, independently verifiable data to demonstrate that god does not exist. It is this one characteristic of the nature of the claim of god that allows so many to derive a sense of validity to the claim. Mikey himself has challenged at least once "prove that there is no god". Well, the great answer to that challenge is that I can not, and I freely admit that. I can not disprove god. The existence of god is, by its very nature, unfalsifiable. This, however, does not give the claim of god any validity. There are literally an infinite number of concepts which I can devise that carry the same characteristic of unfalsifiable. Famous example used to demonstrate how unfalsifiable does not automatically afford a sense of probability: ever heard of the flying spaghetti monster? Probably - and you know what, you can not disprove the flying spaghetti monster.
The universe is incredibly mysterious. There is no telling what the investigation of its origins will lead to. But when you simply believe that the origin of the universe is "god did it" while at the same time asking for no evidence to support said claim, you are effectively eliminating the motivation to investigate the true origins of the universe. And it is not just the origins of the universe that have suffered at the hands of the "god did it" claim. Evolution, the most important concept to come along in the field of biology and has produced everything from the development of vaccines to the ability to write digital data to discs with lasers (yes...the study of the evolution of the eye of a moth lead to the development of CDs). Yet, evolution is continually the subject of controversy, is still battled with in school boards and even federal courts, because people have taken the affirmative to the claim that "god did it", without even the slightest bit of objective evidence that shows those claims to be true, even in the face of the extraordinary amount of evidence that evolution is indeed true (there is more evidence in support of evolution than there is for the existence of gravity).
When asking "why atheism", ask yourself this question: why "your" god? There are thousands of gods, past and present, that have been worshiped. The reasons for people's differing religions are extremely subjective. Most of the time it is simply the religion that their parents followed. At the heart of all religions is the existence of a deity that is responsible for the creation of all. This is a claim that holds two possibilities: either it's true or it's not. However, when have you ever known any explanation for real world observations (such as the existence of the universe) to be dependent upon personal creed? There are hundreds of different religions...they can not all be right, but they can all be wrong, and, as of present, no evidence has been presented to verify the validity of one religion versus another. So, why "your" god? I am not trying to be rude or mean to people who believe in god, I am simply trying to ascertain the reason for the belief. I have gone through great detail to explain my atheistic position to the claim of the existence of god, and the questions I have posed here are to help those who do believe understand that it is because of these questions that have yet to be effectively answered that I will always take the skeptical position to the existence of god.