JS-POG hit a lot of good reasons. I choose to be an atheist (ish) because it seemed the most likely world-view to be true. Others have said similar things, but that it wasn't then a choice, but I'd have to disagree. With every decision like this there is a certain amount of uncertainty. And with that there is well, wiggle room. You can reserve judgment, bank on undiscovered details, or simply stick your head in the ground and hope it isn't true. I've got a pretty quote for that:
And in the end, I chose this belief because I thought it would make the world a better place. Rationalism for the win.
What now? We know all sorts of stuff. We know more then one person can ever hope to learn in a lifetime. As for "what was around before the big-bang?" welllll, yeah, there are gaps in our knowledge, but we never claimed to be omniscient. Look at it as a challenge, if you discover something new, maybe they'll name it after you and people will be measuring the juvenissun's of a whatnot gizzmo.
Of course. To an extent. But every piece of evidence and positive test lends weight to believing in it. I believe that the sun will rise tomorrow because it always has in the past, I understand how the planets turn, and I don't foresee any cosmic event likely happening that would stop it. The basis for science is skepticism. If that disturbs you or something, perhaps I can ease your mind by saying that some things are so fantastically improbable that it's not even worth considering. Now, this may bite you in the rear if something truly fantastic happens, like Shiva descending on a pillar of light and saying hello or some such. But it certainly makes life simpler.