So far as philosophical questions go, surely none are as capable of warping the boundaries of the mind as this one. I've quite literally lost sleep pondering this question over the years and I'm not sure that this lost sleep has brought me any closer to providing a definitive answer. I was planning on leaving this question for later on, hoping that my writing would - with practice - develop the scope and precision necessary to do a question like this justice, but then I figured that such is a day is unlikely to arrive any time soon, so why not just jump right in?
Okay then. Here we go.
First, a word on the nature of the question itself. In a theological debate, any question involving the word "why" has to be treated with a careful dose of skepticism as it can be used to imply the need for a teleological answer where such a need may not actually exist. For the atheist - who necessarily denies teleological agency as a quality of the universe - it's difficult to interpret a statement like "why is there something rather than nothing?" as anything other than an example of begging the question. By forming the question that way, the theist could be seen to be assuming a priori the existence of the very thing (teleological causality) that the debate requires him to prove a posteriori: in other words, the theist is implicitly assuming that there must necessarily be a "why" at all. For that reason, if such a theological question can be rephrased as a "how" question (i.e. "how is it that there something rather than nothing?") it is generally prudent to do so, as it permits the atheist and the theist to debate on something approaching a common epistemological ground.
Having said that, I still think that this question in its current form warrants a response and given that Williams poses a very similar "how" question next-up ("how did that something come into being?") I'll save the scientific issues for the next post, and focus on the philosophy here.
This question, needless to say, is an old one, dating all the way back to Plato and almost certainly much earlier. In the context of Christian theology, the question has been most famously framed by Aquinas (who posited that the chain of causes that is our universe must have been set in motion by some "uncaused cause") and Leibniz (who speaks of a "necessary being bearing the reason for its existence within itself"): both, of course, are here making reference to "God". I'm not going to address these arguments directly*, but I will use them as a means of demonstrating why such intuitions about the nature of existence are misguided and - ultimately - fallacious.
So let's return to the question itself. In asking why there should be "something" rather than "nothing", there is a hidden assumption: that we should expect "nothing" to be the default state of nature. That is, that without some "uncaused cause" or "sufficient reason", "nothing" would necessarily exist and "something" necessarily wouldn't. Beyond the fact that, by definition, nothingness cannot ever be said to "exist", in reality there is nothing to support this assertion, and all the available evidence actually points to the contrary conclusion.
On the quantum scale, particles in a vacuum (i.e. an "empty" part of the universe) flash in and out of existence all the time without any discernable physical cause and these particles themselves can only be said to exist probabilistically rather than as determinate points of "existence". The only absolute "law" operating at this scale is that any existent state (including "nothingness") must be completely and inviolably indeterminate and therefore, over any meaningful period of time, inherently untenable. I'll address this more thoroughly in the next post, but for now I will simply point out that "nothingness" (i.e. a quantum state that is both unchanging and certain) represents an unstable and - above all - impossible state of nature.**
But let us presume, nonetheless, that the existence of "something" must be dependent on the existence of some prior "something": that is to say, let us presume that all existence is inherently contingent. Let us then presume that the chain of contingency must terminate somewhere: that is to say, let us presume that the chain of causality cannot be infinite and there must be an ultimate "cause" to have originated the chain in the first place. Let us then presume - a la Aqunias, Leibniz and any number of theologians - that this ultimate origin could best be described as "God". Are we any closer to answering the original question?
In the first place, by positing the existence of a god (even one for whom existence is a predicate) we are still no closer to answering why there should be "something" (i.e. a god) as opposed to "nothing" (i.e. no god) and the arguments of Aquinas, Leibniz and others surely offer no solution in this regard. If it is permissible to pull a semantic sleight of hand and suggest that "God" must exist as an uncaused cause (Aquinas) by virtue of its very necessity (Liebniz), could we not with similar effortlessness invoke Ockham's Razor and render any such "god" superfluous by suggesting that the universe itself must exist as an uncaused cause by virtue of its very necessity? Is it really more reasonable to terminate the contingent chain of existence at some god, rather than with existence (i.e. the universe: the sum-total of every "something") itself?
In the second place, even if it could be cogently argued that the uncaused cause cannot be "the universe itself" and must, rather, be some "god", is that any reason to infer that this god must possess anything like the attributes assigned to him by modern theology? That a "god" must exist as an uncaused cause - and the foundation of all existence - is not to say that this god must necessarily be rational, or purposeful, or loving, or even - by any meaningful definition of the term - conscious. Such a "god" could just as naturally be mechanical, incapable of purpose, amoral and completely unaware of its own existence: what clause within Aquinas' and Liebniz' reasoning logically precludes such a conclusion***?
You'll note, as this post becomes more and more difficult to follow, that ontological discussions such as this one speed very rapidly towards the point where our imaginations and capacity to express ourselves cogently are strained to breaking point. While this inevitably leads to frustration and the desire to invoke - quite literally - some deus ex machina into existence as a means of alleviating the existential angst that such a line of questioning will inevitably provoke, I would proffer a more honest summation: we just don't know. That is to say, I don't know "why" there is something rather than nothing, the cosmologists don't know "why" there is something rather than nothing and the theologians don't know "why" there is something rather than nothing. This uncertainty, however, is nothing to fear or to run from: I think it's simply one of those beautiful and inescapable realites of human existence.
So, in answer to the original question (I think this is about as equivocal an answer as I'm likely to give to any of these questions), the most generous conclusion is that no-one knows "why" there is something rather than nothing, the least generous conclusion is that one is just begging the question by posing such a question in the first place. I don't think it's an argument that makes the need for gods any more real, but I will agree that it's "fun" (or "complete torture" - the terms are largely interchangable when it comes to philosophy) to think about.
* Immanuel Kant's succinct response to the ontological argument will suffice for now: "Existence is not a predicate."
** One could then, I suppose, modify the ontological argument to say, "why are quantum fields in a constant state of flux rather than fixed unchangingly at zero?" but that still betrays the unfounded assumption that a mandated fixed state represents a far more likely reality than a mandated unfixed state. In any case, as Einstein sardonically noted, a god that finds itself the creator of such a reality could not be the prescient, teleological god of Christianity, as such a god is reduced to "playing dice" where no specific outcome - including the emergence of human beings in need of a saviour - could possibly be planned from the beginning.
*** Paul Davies (in "The Mind of God") argues that such a necessary being must be rational in constitution, but his reasoning for this conclusion was rather opaque and I've got no inclination to retrieve it from my bookshelf and recapitulate his argument here.