More recently, rationalist has tended to have a meaning closer to its current one, but with strong negative affect associated with it. Peter Drucker, for instance, seems to use it as a term of reproach in "Adventures of a Bystander", to mean the sort of small souled narrow-minded person who thinks that they can be right and others wrong and are allowed to say so because they have reasons for their beliefs instead of having made them up to express feelings but the assumption is that one shouldn't do this because doing it leads to communism, fascism, or other forms of authoritarianism. If people don't have the right to believe what they want then some authority must have the right to tell them what to believe. Traditional conservatives can associate this attitude with communism and other badness. Basically, rationalism is used to mean affiliation with authoritarian regimes who claim the prestige of science.
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).