Evolutionism.
My reasoning is that I prefer cold, hard facts to the hope/belief that something out there might be in control of everything. If you take a creationist approach to science, say fossils, for example, and say God placed them in the Earth to test our faith, your argument defies the basic principles on which science is built, and as a scientist I cannot approve of this as it would nullify all my beliefs and ruin reality as I see it.
My reasoning for that a God cannot exist is based on the same set of principles, and until science itself proves said principles wrong I will continue to apply them to the world, and thus perceive it in a way that makes sense to me. I believe that God cannot exist because nothing can be created from nothing, hence a god would have to be created by a more powerful god, leading us to an inevitable paradox containing an infinite number of gods. The same reasoning can naturally also be applied to say the Big Bang theory (note: the scientific term "theory" is used in the way a layman would use the term "law of nature", not as a layman would use the term "theory"), where a singularity underwent sudden expansion and thus created the universe as we know it, also creating a paradox, but I have faith that science will be able to resolve the latter.
I am not trying to flame people with theist beliefs, I am simply stating my own opinions and explaining why I believe in them.