When someone asks me why I don't believe in God, my response is usually, "It's not that I don't believe in God, per se. I never even got to the point of not believing in Him". There are a couple of other things I don't believe in, which, to me, precludes His existence.
I don't believe in perfection. In half a century of existence, I've never seen an example of it. Nothing is perfect, nor should it be. Back when Persia was still called that, the rugmakers would weave one deliberate mistake into each rug on the principle that only God could make something perfect. Hence the term, Persian Flaw. But the universe, by it very nature, is imperfect. Perfection is what existed before the universe -- except that the words, "existed" and "before" really don't apply. Let me try again. Perfection is nothingness. In the absolute. An absence of both the light and the dark. So, if God exists, it's a good bet he's no fun at parties.
You can probably guess the second item. Forever. I don't believe in infinity. Again, no example to be seen anywhere in nature. It's apparent from the previous paragraph that I believe that the universe began at some point. This belief is not borne of faith. I believe it because I can do just enough math for it to become apparent to me. Everything that begins, ends. So too will the universe one day. In about 5 billion years, the Andromeda galaxy will come crashing into the Milky Way. It would be a great light show on Earth. Of course our sun will be well into old age, becoming a red giant and enveloping Mercury, Venus, perhaps even earth, which will have been uninhabitable for millions of years by that point.
Besides, think about the afterlife. It might be fun for a few hundred years, maybe even a thousand. But even another plane of existence would begin to wear on one if it resembled this one too closely. In order to make it different enough, we'd have to evolve into something as different from us, as we are from the atoms we are made of. You can't think 11-dimensional thoughts with a three-dimensional brain, after all. In essence, the very same thing as death. Though, in this case, I think the simpler explanation prevails.