What makes a person an atheist?

This is just a couple random thoughts and not really an essay yet. 
I think that what makes many people an atheist is the same thing that makes a person a fundamentalist.  That would be a desire to understand everything and if one cannot wrap one's head around something, or reconcile it with seemingly contradictory thoughts, then it must not exist.  Thus, the fundamentalist cannot figure out how to reconcile evolution with Genesis and so rejects evolution.  The atheist cannot reconcile miracles with materialism and so rejects miracles.  Usually, one holds such views from an early age and doesn't alter them much, if at all as the years go by.  However, sometimes one becomes the other.  This happens when one does begin to ask questions and finds that there are not good answers in his current belief system.  From there, he rejects the one and embraces the other, but maintaining that desire to understand everything.  Thus, the fundamentalism is just as strong, just from the opposite end of the spectrum.