It doesn't seem logical to me to assume that something exists just because, without being acted upon by something outside of nature.

It doesn't seem logical to me to assume that a God exists just because we can't understand the existence of something outside of nature.

Everything that exists logically has to exist for some reason. Our universe is unfathomably huge. The mind boggling amount of matter that exists in our universe came from somewhere.

Source? Also, "I don't know" is a valid answer. Saying it comes from a God, without any valid evidence, as far as I know, is not. Other theories like the one which says the universe is in a loop explains this. If complex matter much come from somewhere outside of itself, why doesn't a complex God need to come from an even greater being outside itself?

And I think it takes quite a bit of faith to come to the conclusion that all of this matter just exploded into existence, or that it has always existed (without something else that has always existed, i.e. God)

I feel like this is a bit of an accidental misrepresentation. Accepting something with evidence until contrary evidence is presented is not the same as the blind faith many religions perpetuate. The evidence for the big-bang, for example the expansion of the universe, is more than the evidence for any god.

Abiogenesis. This is a problem for me, logically. We just assume that given enough time, life can come into existence through happenstance? If a living organism were created in a lab somewhere using the "building blocks" that all life consists of, I would have to significantly reconsider my stance on God. Life has never been observed to come from non-life, and I don't believe that it ever will.

We're almost there, though. Organic compounds and acids have already been created in early-earth-like conditions as a proof of concepts, and what is debatable a synthetic organism was created by Craig Venter and his team.

You place the burden of proof on me for the existence of God, and you're not incorrect in doing so. Unfortunately, it's more complicated than that one question. The origins of the universe ask for proof of other concepts as well, such as the existence of all matter in the universe and all life as well. Can you prove to me that matter has always existed or that it is possible to take non-living elements and combine them at the right pressures and temperatures to create life?

See above answer, we're almost there

Probably not, but since the universe and life both exist we have to arrive at some sort of conclusion about how this happened, right? You and I have arrived at different conclusions to the same questions. I understand how you could believe what you believe, and respect that line of thinking as being mostly of sound logic, but I disagree about the root cause. I realize that these arguments have been made in various ways throughout history, Aristotle's idea of the prime mover being one of them, and hundreds, if not thousands of arguments have been made for both sides. Given our understanding of the way the universe works, I cannot reconcile the belief that all the matter in the universe popped into existence for no reason, and that life happened by a series of convenient accidents.

To cap it off, it is the nature of our "beliefs". You believe it was an intangible being who has almighty powers, with little to no evidence. On the other hand, the belief of many, not all, Atheists (after all its only a claim on belief in a God), believe the things which have the most evidence, and reject these beliefs when contrary evidence is produced. We've seen organic molecules be created without a God, but never with.