Let me admit that as a kid, I used to be asking this kind of philosophical questions, too. However, these questions turned out to be so much less interesting than other questions that I threw them away. One may also argue that the question is ill-defined as well as contradictory.
On the other hand, one may also refine the question, give it a well-defined, quantitative meaning, and simply answer it from the first principles. Why is there something rather than nothing? Well, it's because of any of the following reasons ;-)
- nothing doesn't allow intelligent life (I actually find it OK to use the anthropic principle to answer Leibniz's question and similar qualitative questions whose information content is tiny)
- the worldsheet conformal anomaly wouldn't cancel for D=0; note that string theory is actually capable to answer these old questions
- nothing violates a basic rule of physics that can be derived from string theory, namely the existence of gravity in the low-energy limit; this fact is related to the previous one
- nothing violates the second law of thermodynamics because if there were nothing, the entropy couldn't increase
- nothing violates relativity because one can't equivalently describe the phenomena in nothing using different reference frames
- nothing violates unitarity; a Hilbert space that allows both something and nothing to exist is at least two-dimensional while the nothingness Hilbert space is one-dimensional or zero-dimensional and can't be related by a unitary transformation
- nothing violates the uncertainty principle because if there were nothing, all observables would commute with each other (0.0=0.0) and there would be no uncertainty
- the entropy of a zero-dimensional Hilbert space is minus infinity and violates the finiteness of entropy
- while nothing is simple, it violates Einstein's rule that things should be as simple as possible but not simpler
- last but not least, there is also an experimental reason: the existence of nothing has been ruled out experimentally because you have just made an observation of this sentence; let's admit that you were not the first one who has made an observation of something and every previous observation of anything has falsified the nothing hypothesis
As you can see, the question is actually well-defined and the answer is easy to find. It is much harder to answer more detailed questions about the something that exists, for example what is the topology of the compact dimensions in which our world is more weakly coupled than in all others.
Are you dissatisfied with most of the explanations because I had to assume some laws of physics or even string theory? Well, indeed, I have done so. And from a scientific viewpoint, anyone who tries to answer these deep questions while ignoring physics or string theory or while putting some metaphysical prejudices above the laws of physics is a crackpot. He can call himself a President of the Philosophy Division, too. It's a matter of terminology and social conventions, not science. ;-)
There exist other answers to the question that I believe are incorrect, for example:
- nothing would violate the scientific consensus because no one would agree with it
- nothing would violate the catastrophic global warming theory: if there is nothing, there can't be any cataclysm either
- nothing discriminates against women because if there were nothing, there would be no women in physics
- nothing is a bad theory according to Lee Smolin's criteria because if there were nothing, no new breakthroughs in science could be fully completed for more than five years
- nothing violates the newest versions of loop quantum gravity because there are no stinky octopi that can swim in nothing
- nothing disagrees with doubly special relativity because if there were nothing, there would be no proof of the inequality defining the soccer ball problem either; that would contradict the rule that all doubly special relativistic researchers only study theories where soccer balls can't exist
- nothing would still suffer from the soccer ball problem because there would be no soccer balls
- nothing is wrong because God is great and if He had created nothing instead of something, He would have been a loser which is a contradiction
- nothing is not even wrong
- people who believe in nothing are deniers who are paid by oil corporations
Some of these answers are flat out wrong because they are based on incorrect assumptions and invalid equivalences while others don't answer the question "Why is there something rather than nothing" but instead attack the person who asks the question or fill the readers with sociological and religious propaganda or emotions that have nothing rather than something to do with answering of the question.
And that's the memo.