I wanted to expand on an idea I went into a little earlier, I think that there's more than just a difference between an atheist and a believer (we'll leave out the inbetween people but this applies to them too), I think better understanding could be acheived by thinking of these terms in duplicate - emotional vs rational atheism and emotional vs rational believer. (As I said, to keep it simple we'll leave out agnostic, but same thing applies, emotional vs rational).
I think that what leads to those people who have such excellent fulfilment and a sense of purpose and belonging (insert other good stuff) is those people who align both parts of their mind.
Sometimes people go from atheist to believer, and from believer to atheist (note though, this is important. Most atheists are not like myself, having never really believed, the vast majority were brought up as believers and lost their faith later in life. So when someone talks about atheists becoming believers, it's an important distinction to find out about in their history, which helps understand the "why" of their change).
I think some people try to be atheist when really their "heart" just isn't made for it. It leaves them feeling empty, and eventually they end up going back to some form of faith. I don't really know why they try to be atheist, I think some people get overwhelmed by how science contradicts a lot of literalist interpretations in religions, and instead of just revising their beliefs to fit with the science (or as some people do, revising the science... I hear there's a nice museum in the South of the US that tells all about how we used to live with dinosaurs...) they pretty much have a crisis and bam, soon they've got themselves convinced that their now atheist.
I'd hate to be in that situation, and I'd absolutely end up back believing in something, because we have to be emotionally satisfied.
Trying to remember where this was going, ah yes - neither the "conscious/rational" part of our minds nor the "subconscious/emotional" part (yes that's a goofy way to split up the mind, but it illustrates my point!) is more important nor necessarily more "correct" than the other. How many times has our subconscious and emotions saved each of our butts and proven to be "smarter" than our conscious mind? Same goes for the rational/conscious part.
I have no idea why some people's rational mind looks at the data and says "yup, proof of a creator" and another person sees nothing of the sort. I also have no idea why some of us are deeply convicted emotionally (with or without the cooperation of the rational part of the mind) that there is a creator, and why some of us are equally (we assume equally anyways, don't exactly have a hydrometer for "conviction"

) deeply convicted emotionally that there isn't one.
If anyone knows any research or deep thought that's been written on the subject I'd love to read it - because here's the truth: none of us is really ever sure which part of our mind is really in charge of what. For all I know my emotions (not wanting a creator) twist my logic to suit, and for all a religious person knows the same might be happening to them. That's not the kind of admission many might make about themselves, but I'm pretty dedicated to truth. It's frightening how little access we seem to have to our own minds sometimes.