I cannot speak for others, but I'll do my best at getting my thoughts across.
Scientific discovery after scientific discovery has shown that the universe is, by nature, an objective construct. It's governed by logical rules through and through. Everything that happens, microscopically or macroscopically, happens because the rules of biology, physics, and chemistry prescribe that they happen. Even the idea of consciousness, and even free will, are beginning to be revealed as governed by objective rules.
In this massive conglomeration of objectivity, I see no place for the existence of an all-powerful being that watches over us, cares for us, lends us passage to the afterlife, etc...
I just feel the concept of God is nothing more than an archaic remnant of early civilization's attempts to explain the unknown. For whatever reason, perhaps because it provides a sense of solace for people encountering hard times, God has lived through the ages.
Civilization created God, not the other way around.
As we learn more and more about the Big Bang and the existence of possible multi-verses, the likelihood that some sentient being created our universe seems ever less likely.
I guess the difference between me and a believer is faith, faith that despite no supporting evidence, God does, in fact, exist and watches over us all. You know what? Maybe the believers are right. I think they are naive but maybe they're right. I'm just not the type of person to allow my faith, or lack thereof, to shape my view on life, existence, the universe, etc...
People say you need faith. You just have to believe he's there. You can't proove he doesn't exist so if you believe, you might be right. The problem I have with that is that you can say that about just about anything that doesn't exist. For example, let's say I believe there is an eight-legged monkey that lives in the Congo. Even if it doesn't exist, I can still maintain that I've seen no evidence contrary so it must exist.
I'm a man who deals in numbers, in formulas, in facts....in objective truths which I can perceive and understand. There is no place for a sentient creator of the universe in my world of objectivity.
With that said, I don't think religion is wrong. I think organized religion is wrong. It only purports the idea that there is one way to "enlightenment" as opposed to the "wrong" way offerred by other religions. Still, the teachings of the major religions of the world all have fantastic messages...and it just so happens those messages all boil down to one concept...Love. That is as marvelous as message as any and I am all for the religious teachings (be it from the Bible, or the Koran, or the Talmud) that try and spread the message.
It's just when that when the message becomes entangled with "devotion to Jesus" or "devotion to Mohammed" or "the TRUE path to salvation", then I take offense.
Quick note: America may have been founded in a Protestant environment, but the morals and values taught by Protestantism were around much earlier than Protestantism, itself. The morals and values themselves are very secular, despite the religious upbringings of the Founders. The fact that churches, synagogues, mosques, and other places of worship flourish side by side in America provides pretty good evidence for the success of separation of church and state. I guess that doesn't make America an atheist nation, per say, but it does make it secular, in that rather than affiliate itself with one religion, it affiliates itself with no religion and, in turn, becomes a civil nation.