Why is there a God?
There a billions of stars in a galaxy and millions of galaxies in the universe. Many of these stars aren't even medium yellow like our sun. The odds of a planet forming to Earth-like standards are actually microscopic, even if other planet types can form the basics of life. Our planet is one tiny speck in a vast universe. I would almost call it insignificant.
What makes you think we're the center of some grand cosmic scheme of some importance?
If God is so powerful, why didn't he make a universe with many yellow suns, complete with at least one green Earth populated by human-like creatures?
If there are others out there, why didn't he make some way for the other planets to become aware of each other and communicate with each other?
We're the only planet that we know of that contains life and that's supposed to be proof of a god? I'm not impressed.
We came DEAD LAST in the history of life. Two seconds to midnight as it were. Many lifeforms that came before us look like aliens. If there is a god, why should we assume that humans are made in such a being's image?
You got many things wrong in the past. The shape of the world. The planet's position in the solar system. What makes you think there aren't other things you're getting wrong?
You've killed people for holding scientific views, philosophical views, writing books on said views, members of other religions, and even yourselves for variations of your own freaking religion. If your religion makes you moral and those were "different times", what's stopping you from going back to the old habits, and why should I trust that you won't?
There are/were thousands upon thousands of deities and beliefs created by humankind throughout history and into today. Most of these gods are now dead as being mere mythologies. And even believers reject the gods of everybody else but their own and draw lines on what they believe and what they write off as nonsense.
Why does there have to be a god? Why does it have to be YOUR god?
Why is the atheist immoral for doing exactly what all believers do naturally but going one step further?
Why should I trust your morality that is derived from your religion because you require a special instruction manual to tell you how to live your life but not the morality of an atheist who does things because his heart tells him these things are good?
What happens if all your instruction manuals suddenly disappeared from the world?
You've been waiting almost 2,000 years for your god to return in human form on Earth. What do you tell your date who's late for 2,000 minutes(and counting)?
Science describes the world in terms of matter and energy. Nothing in there about magic. Both are things we can detect and measure. You reject the existence of vampires, leprechauns, and unicorns. Well some of you do anyway. You've yet to see an example of somebody levitating, casting fireballs, or giving actual curses that wasn't some parlor trick or the power of suggestion.
Why do you still believe in angels and demons?
Why do you still believe in witches?
Why do you still think religion is winning?!
If there is a god, and he made humans to be in his image, why did he make me human only to charge me with being guilty of the crime of being human, the sentence for which is eternal torment?
If God is perfect, why did he create such a screw up of a project and then go on to blame the screw up for its existence?
If your vision is such an absolute truth, why are humans given to creating such a wide variety of belief systems? And why do I have to pick your particular flavor of world views because all the others are wrong and won't save me from the crime of being human(the sentence for which is eternal torment)?
Why is this a "loving" god now?

And why won't you leave me alone?

Why. Such a powerful question. When you ask it, you have to find the answer or at least attempt to find it. Hopefully you find the answer through your own actions and with your own eyes. Instead of, you know, accepting what a book tells you.

I guess I sort of turned this into a poem. O_O