My reasons for being an atheist are:
1. The concept of hell in Christian and Muslim theology. Jesus said, "Narrow is the way" so evidently most people are going to hell. Infinite excruciating punishment for finite sins is not only atrocious, it is illogical. This argument only applies to Christianity and Islam, however.
2. Natural explanations have been found for practically everything, making God superflous. There are natural explanations for the origin of life, Evolution explains the diversity and complexity of life, and of course there are natural explanations for the origin of the Universe and its laws.
3. The Ultimate Boeing Jet 747. Creationists argue that the odds of life happening are similar to a tornado blowing through a junkyard and producing a brand new Boeing Jet 747, therefore life must be designed. I have read, and fully believe, that the human mind is the most complex structure in the universe. Since human beings cannot create life from scratch (yet) much less a Universe, I conclude that God must be exponentially more complex**. Since, according to the argument, complex things are highly improbable, God must be even more improbable than the Universe. Normal rules of logic tell us that we must choose the most probable option, all things considered. Therefore, God does not exist.
4. I know of no evidence or arguments for God which do not have major flaws.
5. Supposing that there were one God or Pantheon, why is it that different cultures all worship different Gods or Pantheons?
** I am assuming that I can make an analogy between what is inside the Universe and something that is supposedly outside the universe. If human beings cannot make that analogy, then there is no way to know whether there is a God or Pantheon, or which one it is, and we simply have to say that we do not know.