I'm typing this quickly, so if I don't say things just right, please forgive me.
Personally, it's hard for me to conceive how atheism can develop w/o being--to some degree--a response to theism. Most of us have likely grown up in a societal context where a religious element did exist; and there may even be some innate tendency to develop religious belief systems, as evidenced by prehistoric rituals which strongly suggest an awareness of possible life-after-death.
That aside, it seems to me that atheism comes about for 1 of 2 main reasons:
1. No proof, specifically no empirical proof of "God." IMO, insistence on empirical proof for a nonempirical entity raises its own set of problems; nonetheless, a critical thinker can arrive at an atheistic conclusion relatively independently of the dogma he/she was raised in.
2. The statement may take various forms, but the essence of it is this: "If there is a loving God, why is there so much suffering in the world?" Here is where, I think, we find the more "radical/vehement" atheists who, instead of reaching their conclusion mainly through Reason #1, are reacting more from anger, resentment or disappointment.