When Pierre-Simon Laplace presented a copy of his Celestial Mechanics to Napoleon, and Napoleon asked him why such a large book about the universe made no mention of a Creator, Laplace is purported to have replied, “I had no need of that hypothesis.” This response encapsulates, in the simplest, most general terms, why I am an atheist: I am an atheist because the hypothesis of a god is completely unnecessary.
Our understanding of the world at least since the Scientific Revolution has moved steadily in the direction of increasing naturalism: with each passing year, science has brought under the provenance of natural law more and more phenomena formerly attributed to the activity of spooks, leprechauns, vital forces, and gods. With each passing year, science has given ever greater indication of the sufficiency of nature unto itself and its operation as an unbroken system. So strong has this trend been that naturalism now justly is the default position of the informed.
I do not, by any means, claim that science is finished, or that there are no phenomena that are deeply resistant to scientific inquiry. However, where questions or anomalies still exist, supernaturalism does no better a job than naturalism at answering those questions or explaining those anomalies. Investigate for a moment how the introduction of a god or leprechaun is supposed to provide an explanation for the gaps that still exist in our knowledge, and one quickly finds vacuity covered with a thin veneer of special pleading. To summarize: what we understand of the world requires no god, and what we do not understand of the world is not helped by positing one. The hypothesis of a god is utterly superfluous.
Of course, those who are familiar with the tale of Laplace and Napoleon will know the sequel: when informed by Napoleon of the exchange, Joseph-Louis Lagrange opined that the hypothesis of a god was a very fine hypothesis, and explained much. I mention this simply to state the obvious: that what I have offered here is intended only as the first word on the matter, not the last.