(NB: Since first writing this I've come much more to the live-and-let-live frame of mind, and recognise that religion can fill many important roles for some people. But my claws I keep sharpened, because some people just don't live and let live.)
I have two answers to this question, which I give depends on an evaluation of the questioner's intelligence and how obnoxiously evangelical or persistent they are. I'll give the pat answer first, and then follow up with a better explanation. Feel free to skip the first one, it's insulting.
This answer is broadly targeted at the many theists who seem to think that there must be something wrong with me because I don't believe in their god, religion, cult or giant electric loch monster. After all, the magnificence and munificence of their whatever is clearly so obvious to all right-thinking people that you would have to be blind, stupid or insane (or all three) not to believe.
It's about here that I pull out my trusty clue stick and wave it about. I figure there are roughly 10,000 religions in the world. That's a conservative figure, but it's also one I pulled out of my posterior. Feel free to substitute any other number greater than one.
How many of these do you believe in? Do you worship Allah? Christ? God? Gaia? Buddha? Xenu? (Or whatever those wacked-out Scientologists worship.) What about Shiva? Zeus? Ra? The Invisible Pink Unicorn? Binky? (The one and only 800ft carrot.) What made you pick your religion over everyone elses'? How certain are you that you picked the one that is actually true? More importantly, why are you that certain? Do you have any proof? Or do you just hope?
One weak objection you might hear to this argument-by-probability is the statement that there is in fact a single underlying and all-encompassing world religion. This is despite the fact that these supposedly unified religious groups frequently and eagerly declare Jihads and Crusades on each other. And if it is still the case that there really is only one world religion? Then they've been committing fratricide on an indescribably grand scale for the past 10,000 years. Would you want to belong to an religion that regularly tortured or ostracised its members under the pretence that those being tortured belonged to another religion? You think that just maybe there is a reason that suicide cults tend to be shortlived? (Above and beyond the obvious reason of course!)
Allow me to spell my point out as bluntly as humanly possible. If you accept my figure of 10,000 religions, and that, by and large, they are incompatible, then you are an atheist with respect to 99.99% of the worlds' religions. I'm a nice round 100%. Do you really want to play the odds that your 0.001% just happens to encompass a God that actually exists? And if so, what the hell do you base this on?
Hence, I rest my case: We're all atheists. I just happen to be more honest about it.
This is the internal rationalisation I make to myself, although intriguingly, over time the 'mock' argument I just made becomes more sensible sounding. This reason however ss my reason for being an atheist, not the one I fob off annoying people with. It also happens to be the one that gets debated whenever I seriously discuss the issue. If you'd like to comment, please address this explanation. Furthermore, note that I have written this with respect to the Judaic faiths, although it may - and probably does - apply to many others. Broadly, this reason, treated by me up until now as a single reason is actually comprised of three:
Traditionally the work of an argument rests with the person making the assertion. This is essentially an axiom - something assumed - but lets conduct a gendanken about an argument in which the argumentative burden is a burden of disproof. That is, if you suggest something, it is my responsibility to disprove it, otherwise we assume it's true.
You say God exists. I say the Invisible Pink Unicorn and Binky exist. I also claim that gravity is an illusion, created by the Illuminati to aid them in their plans for world domination, that I am a vampire, that Martian crocodiles have green blood and that gremlins are responsible for all those airplane crashes. You haven't a hope of disproving any of the above - I can always come up with an ad hoc alteration to my theory to maintain some form of consistency with reality. The obvious, and only sensible, conclusion then of this very brief thought experiment is that it is only reasonable that it is the theist's responsibility to prove their god exists, and not the other way around. Atheism is the default position.
The second aspect of the burden of proof is what is known as the “god-of-the-gaps” issue. Traditionally, anything we can't explain is attributed to the god(s). The flaws in this particular viewpoint are pretty damn obvious, but let's be boring and explicate them.
One: To paraphrase the common saying, back in the stone age, the gods were just outside the lights of the campfire, but now that we have halogen lamps, they've buggered off over the horizon. Sure there are some gaps left at the moment. But those gaps are shrinking, and so is the once all-powerful, all-knowing and all-loving god that fits in them. How long will it be until God is less powerful, less knowing and less loving than an ordinary human? And would God still be a god?
Two: The alternative explanations objection. Imagine we were out camping in the forests, it's late at night and we here strange rustlings in the treetops. Cowering in fear, I proclaim that the noise is made by a gang of serial axe murderers who work together to slaughter innocent trampers. I claim that, “since you can't come up with an explanation I must be right”. Would you accept my line of argument? What if I claimed it were aliens, collaborating with elves and angels, in order to take over the world? I think it's clear that a lack of an alternative explanation neither proves nor disproves another hypothesis. “Goddidit” is not a satisfactory explanation, regardless of the existence or non-existence of alternative explanations of the various “miracles” that occur.
The issue of worthiness is also something I am surprised theists take for granted. Please understand here that I refer to the worthiness of God to be worshipped, and not our worthiness to worship. Does God (or the gods) really deserve our worship?
Taking examples from the Christian and Jewish faiths, we can note that this is the God that smites entire cities, and turns into salt anyone who would dare to try and witness the act. (Sodom, Gommarah and Lot's wife, respectively.) This is the God that “divinely inspires” laws that compels the victim of a rape to marry her rapist (at a cost of 50 shekels) and condemns her to death if she fails to cry out loudly enough (Dt.22:23-34). The Skeptics Annotated Bible has literally thousands of such examples. Is such an entity really worthy of our worship? Does a being whose efforts best Hitler's thousands of times over deserve our bowing and scraping? Leaving completely aside the morality of the idea that you can torture someone eternally simply because they disagree with you (is it moral for me to torture you, even briefly, if you claim I don't exist?) there are some pretty significant ethical issues here. Significant enough that worshipping such a god is not something I could actually stomach.
Finally we come to the problem of evil. Simply put, the Judaic faiths claim the existence of a God that is omni-potent, omni-scient and omni-benevolent. In other words, all-powerful, all-knowing and all-loving. Needless to say, if he really was all of the above, then all the unnecessary evil that we see around us - the disease, the disasters, the plain, simple bad luck - shouldn't exist. If you believe in the existence of evil (with or without a capital 'E'), you'll have problems with a Christian God.
It has not been my intention to cover any of the above issues in depth - namely the arguments for the existence of god, the moral issues, nor the problem of evil. Each is easily a book length topic, and even a brief overview - of which there are many on the 'net, all better than I could write - would be longer than this essay. But the arguments alluded to in this brief essay are the reasons I don't believe in a god, and I wouldn't worship it even if it did exist. [