I would never have identified as a rationalist had I missed this site. I never had a very strong commitment to the truth, as I am something of a chronic liar. I used to make deliberate attempts to try and manipulate people in ways borderline to the Dark Arts.
I did however desire to have a consistent set of philosophic rules that eventually led me into an existential crises of sorts. I was raised by a deeply conservative (in ideology, but certainly not action) father who is easily the smartest person I know personally at the moment. He was intelligent enough to defend his own biases. I ended up believing in intelligent design argued from an almost logical point of view.
I became an objectivist for a short time, and followed the ideas presented to me to their logical conclusion, giving me my first taste of rationality. unfortunately i then decided to study philosophy and based many of the ideas I developed subsequently on internal reasoning rather than observation. This led me dangerously close to postmodernism (shutter), without realizing it.
I never cared much for science, being raised with moderate distrust of scientists who, i was led to believe, arrived at irrational and convoluted conclusions to promote politically-inspired ideas (proven by the ridiculous borderline pseudoscience the media tends to get a hold of). I also came to possess a highly contrarian attitude and abhorrent social tendencies (i get along fairly well with most people but often demonstrate quirks that lead several people to label me a sociopath. Think of the guy in the hat from XKCD only more realistic).
I came to be a rationalist after reading the chapter in HPMOR in which Harry first conjures a patronus. I had ceased to believed in an afterlife long before, but never considered the idea that death could be stopped. Unlike most people, i noticed that a lifespan of 100 years, being less than nothing in cosmic terms, rendered human life almost insignificant. This ironically led me to lose any value i put in my life and seriously consider suicide.
When i realized that humanity had a fighting chance against death, I regained the will to live and a purpose to strive for, which inevitably led to seeking out any tool that would help. Of course, having gotten the inspiration from Eliezar in the first place, i returned after reading the fic to learn all i can about "what Rational!Harry knows and then some" and discovered the sequences. So it was that I became a rationalist, not out of moral commitment to truth (I developed that later), but out of need for a weapon with which to destroy evil. I learned directly from Eliezar that the strongest weapon man has is the ability to locate truth.
To break up the awkward silence at the start of a recent Overcoming Bias meetup, I asked everyone present to tell their rationalist origin story - a key event or fact that played a role in their first beginning to aspire to rationality. This worked surprisingly well (and I would recommend it for future meetups).
I think I've already told enough of my own origin story on Overcoming Bias: how I was digging in my parents' yard as a kid and found a tarnished silver amulet inscribed with Bayes's Theorem, and how I wore it to bed that night and dreamed of a woman in white, holding an ancient leather-bound book called Judgment Under Uncertainty: Heuristics and Biases (eds. D. Kahneman, P. Slovic, and A. Tversky, 1982)... but there's no need to go into that again.
So, seriously... how did you originally go down that road?
Added: For some odd reason, many of the commenters here seem to have had a single experience in common - namely, at some point, encountering Overcoming Bias... But I'm especially interested in what it takes to get the transition started - crossing the first divide. This would be very valuable knowledge if it can be generalized. If that did happen at OB, please try to specify what was the crucial "Aha!" insight (down to the specific post if possible).