The reason why I'm an Atheist can be summed up very briefly: I have never seen any evidence that any God or anything supenatural exists. As well, I have never heard a coherent definition of God, so he word is meaningless to me.
When it comes to God, I flat out deny that anything like that exists. I started out as an Atheist when I was born, then because of societal influences, spent most of my life as a Believer in the Christian God, to finally give up on this belief to become an Agnostic, then a Weak Atheist, and finally I became a Strong Atheist -- or as one of my friends puts it, a hard-ass in-you-face Atheist. I don't deny it, it's true!
Some people, Mostly Believers, think that because I deny the existence of their God, I have to provide proof that no God exist. Not so, I don't have to prove anything to Them. They're the ones making claims as to their God's existence and providing absolutely no proof whatsoever that He exists, so why should I be put on the spot? I'm merely rejecting a claim they have yet to prove.
To my Agnostic friends I say: Come out of the closet and admit that you're Atheists. After all not one Agnostic believes in a God, so that makes them, at the very least, Weak Atheists, so why not just take that extra step and be done with these myths once-and-for-good?
The funny part is that my friends are not agnostic about faeries, little forest pixies, leprachauns, gnomes or Santa Claus, so why are they agnostic about God? Maybe it's because they're afraid of the stigma attached to being an Atheist, I don't know, but the Agnostic position is, no offense intended, fence-sitting.
You Theists on the other hand, you need to get your act together and come up with some verifiable evidence. No philosophical or theological gobbledigook, but evidence we can verify. My take on this? We'll never see this evidence because God is just as airy and mythical as Mother Goose.