"If you want to know if you're insane, ask yourself if you have an unwavering belief, one that you could never disavow no matter what. If you answered yes, then you're insane." -Ignots Pistachio
I arrived at atheism rationally, rather than through any sort of repression, guilt, anger, etc. Nothing special "happened" to make me start my journey---no abuse, no friend's/relative's untimely death, no imposed "Calvinistic" lifestyle, and no other type of traumatic event. I was raised immersed in Christianity---family, friends, schools, etc.---even a grandfather minister. I had a good upbringing, good parents, good friends. I wasn't forced to accept certain things, and my parents were always supportive. However, I was quite sheltered; throughout my upbringing I was presented, as fact, almost (but not quite) exclusively with a viewpoint of the world that I now consider to be very skewed. That viewpoint was religion in general, with Christianity as the particular representative religion (and Seventh-Day Adventistism as the denomination).
I suppose I've always been rather "scientifically" minded, i.e. rational. I've always excelled at most things, but math/logic was especially stand-out at a young age, as compared to other kids. I appreciate the precision and neatness of reason---the fact that, if one is careful, new facts may be deduced from old ones, thereby expanding knowledge (in a way). In short, I find mathematics and science interesting.
I grew older, and the more and more I learned and thought about things, the more and more apparent it became just how stupid (any/all) religion is. As Sam Harris has summarized, "...religion remains the only mode of discourse that encourages grown men and women to pretend to know things they manifestly do not (and cannot) know [not even in principle]." What's even worse is that religions repeatedly oppose, and outright deny, factual findings of science, which we can know (at least by any meaningful definition of the word---as much as you can "know" you even exist).
As time has continued to pass, I have become increasingly convinced of the paramount importance of truth, as revealed through a ruthlessly honest evaluation of evidence, as well as through the careful use of reason. Yes, I believe objective truth exists; and, I think anyone with a synapse must agree. (It is possible to be wrong!) Furthermore, I think evidence and reason are the only possible ways we can discern that truth from falsehood. (If there exists some truth that is not evidence-based, I cannot concede its merit. This is because we can't identify it; we can't reliably sort it out from all the false stuff! In effect, it might as well be false.)
Consider a newborn baby. Really consider it. What is the entire extent of every baby's "truth-finding toolbox"? The only reasonable answer is: its own senses and mind. Parents may help guide its progress, but it is not reasonable to accept as absolute truth, totally without question, the word of any person (or book)---including parents and friends, well-meaning as they may be. We all make mistakes, and the only way we can ultimately resolve truth from error is evidence. The more evidence and reason that accrues in support of a particular viewpoint, the less real choice remains in "opting" to accept that position. Beyond a certain point, it becomes absurd to hold any other view; the evidence essentially forces one to "believe", regardless of what anyone has historically (or contemporaneously) believed. The only "alternative" to this is to choose a belief in conflict with the available evidence, which is equivalent to making a conscious decision to believe something false (whatever that means).
So, in a way, well-grounded beliefs are associated with a degree of inevitability, an actual lack of choice. This, of course, makes perfect sense when you think about it: you're honing-in on the real truth, which is the only actually correct thing to believe in the first place! The whole point was to find it, so it should come as no surprise that your options narrow as you get closer to it.
I do not mean to imply that everything can be proven by evidence and reason. But, I do think they weigh-in on everything to varying degrees, thereby generating a spectrum of "probability of correctness"---everything from almost-certainly-false (counter-evidence, or at least no supporting evidence) to almost-certainly-true (massive quantities of mutually-supportive evidence). So, clearly there are different amounts of "wiggle room", given different beliefs. But, the interesting thing is that it is evidence that determines the amount of wiggle room! And, the more wiggle room there is regarding a belief, the less reason there is to be dogmatic about it (i.e., the less supporting evidence there is for its truth). Similarly, the less wiggle room there is, the more certain the belief. This is why I think evidence is so vitally important, and this is exactly the way in which the whole scientific enterprise operates.
(As a side note, I also think there exist things that are basically meaningless to try to talk about in terms of "truth" and "falsehood"---or at least to try to do so objectively. In other words, there are subjective "truths", too. For example, one person may like a particular painting or piece of music or type of food or shade of color or many other things, and another person may make completely different selections. Does this mean one of them is inherently wrong? I certainly don't think so. It's a matter of subjective choice, not objective truth.)
It is important to avoid becoming dogmatic about things simply "because you feel they must be true". Human emotions are fickle, and intuition can be wrong. Simply believing something---no matter how devoutly---does not cause it to be true; belief does not manufacture reality. (Take a stroll through a mental asylum, and note the fervent beliefs.) Similarly, as Aldous Huxley noted, "Facts do not cease to exist because they are ignored." We humans have a powerful innate tendency, known as confirmation bias (amongst many other names), that causes us to strongly favor belief of things that we want to be true. A very common example of this is how we evaluate data within the context of our current set of beliefs. Data that seems to support us is unconsciously assigned greater credibility than data in opposition. Again, that's why it's so important to depend upon the external reality of evidence and reason, and to be as brutally honest with ourselves as we can be. We must constantly probe our own beliefs---and, anything that agrees with us, we must question doubly (so we can be certain we are accepting it on its own merits, rather than "because we like it"). In fact, one of the most effective ways of improving our certainty in our beliefs is to try our very hardest to disprove them, and fail. (Again, this is a huge part of how science operates.)
Basically, we must be willing to admit when we do not know, or are flat-out wrong, and to relinquish beliefs in the face of counter-evidence or lack of evidence. This means we can never be totally certain of anything---but, a claim of absolute certainty is an extremely foolish position to adopt, anyway. To do so is to shutdown self-critical analysis and thoughtful questioning, which I think is important for a curious, healthy mind. But, perhaps even worse, it is equivalent to making the ludicrous claim of omniscience; the implicit message is, "All relevant information is already known, so the truth is known, no matter what future evidence may come to light." And, I refuse to do that. I think evidence is the pathway to truth, and I will not close that pathway. Because of this, I will always remain just short of certain of my beliefs. I will leave room for future evidence and the potential for future belief-modification.
When you think about it, it is totally ridiculous to carry-on pretending something is true in spite of the evidence. Just what are you trying to accomplish by doing so? Who are you trying to impress? And, how the hell is consciously choosing to believe a falsehood going to impress anyone, anyway?! Isn't it far more productive to just admit and accept the truth? I think so. And, I now have way too much respect for evidence, reason, and intellectual honesty to seriously entertain belief in any god(s).
"It is far better to grasp the universe as it really is than to persist in delusion, however satisfying and reassuring." -Carl Sagan
"Even if the open windows of science at first make us shiver after the cosy indoor warmth of traditional humanizing myths, in the end the fresh air brings vigor, and the great spaces have a splendor of their own." -Bertrand Russell
"If there is a God that has special plans for humans, then He has taken very great pains to hide His concern for us. To me it would seem impolite if not impious to bother such a God with our prayers." -Steven Weinberg
"The only excuse for God is that he does not exist." -Stendahl