The perpetual dumbing down of American society is manifested most vigorously in the theological ignorance on display on fora such as this. That makes people ripe to be seduced by a gamut of ideas. The atheist turned Christian philosopher G.K. Chesterton comments “When men choose not to believe in God, they do not thereafter believe in nothing, they then become capable of believing in anything."
Having said that, the two greatest motivations for unbelief are the desire to maximize personal autonomy and the fear of ultimate authority. Two quotes to summarize.
“I had motives for not wanting the world to have a meaning; and consequently assumed that it had none, and was able without any difficulty to find satisfying reasons for this assumption. The philosopher who finds no meaning in the world is not concerned exclusively with a problem in pure metaphysics. He is also concerned to prove that there is no valid reason why he personally should not do as he wants to do. For myself, as no doubt for most of my friends, the philosophy of meaninglessness was essentially an instrument of liberation from a certain system of morality. We objected to the morality because it interfered with our sexual freedom. The supporters of this system claimed that it embodied the meaning - the Christian meaning, they insisted - of the world. There was one admirably simple method of confuting these people and justifying ourselves in our erotic revolt: we would deny that the world had any meaning whatever.” ---Aldous Huxley
"...I am talking about something much deeper–namely, the fear of religion itself. I speak from experience, being strongly subject to this fear myself: I want atheism to be true and am made uneasy by the fact that some of the most intelligent and well-informed people I know are religious believers.
It isn’t just that I don’t believe in God and, naturally, hope that I’m right in my belief. It’s that I hope there is no God! I don’t want there to be a God..."