This tired Christian apologetic for the existence of god is frankly unworthy of serious discourse. It is at least 1000 years old, has been posed and answered many times by philosophers and scientists, without the need of a supernatural explanation. And yet the logic of those answers go in one ear of the theistically impaired and out the other without even slowing down in between. To them the question is, in and of itself, evidence of a boogie man in the sky. When they pose it as some kind of self-evident, prima fascia proof of god they are pathetic in their childlike self-satisfaction.
Now don’t misunderstand. As a philosophical question, or as a question that helps explain quantum physics, it’s useful for mental exercise, discourse and understanding. It’s the religious implication, the theist co-opting of the question for an apologetic agenda to which I object.
Religionists who use this tact don't want the scientific / rational answers. They've been tendered; they abound on the internet and in books brimming with secular reasoning. Theists simply want to continue to repeat the question as a justification for their "God did it" explanation, which is their explanation for everything. They like to use it as though it were some esoteric mystical mind twister that can only be solved by injecting the supernatural.
It's tantamount to the old "If evolution is true, and man came from monkeys, how come there are still monkeys?" That answer, as simple and as obvious as it is to people who understand evolution, natural selection, random mutation etc., and as often as it has been explained to the unthinking creationists, continues to be ignored by them. The question will be asked ad nauseum no matter how many times the gross ignorance it displays has been soundly and completely dispelled. They will continue to believe that is a chink in evolutionary theory's validity, when all it does is make creationists look vapid.
So, following Occam’s razor that states the best answer is the simplest one; the answer to the Christian's questions of “Why is there something rather than nothing? What is your explanation for why we exist?” is this:
A) Because natural phenomenon happen to create life under proper circumstances; and
B) We exist for the same reason that life likely exists on a multitude of planets in the universe; refer to "A" above.
And as a default answer:
C) There is something rather than nothing, because if there were nothing you couldn’t pose your insipid and hackneyed question.