All the reasons for not believing conspiracies are where most conspiracists start. I dismissed conspiracies for years, saying that, yes, people get together for financial purposes and it may look sinister/conspiratorial, but it's not like they're dressed in black cloaks in dark rooms planning out all forms of nefarious misdeeds (like 9/11)! Everyone somewhat calculates the number of people needed to be involved and the chances against one of them speaking up. Everyone initially defaults to pedestrian explanations of conspiracies.
I had a friend who believed the Mena conspiracies and had what I remember as an old VHS on the subject. It had grainy images and I found it easy to not take too seriously at all. I was open to the idea that they may be right after all, but I didn't think they could really prove such a thing (with their fuzzy pics). This is how I viewed most conspiracies, coupled with the idea that the one presenting such things was a bit of a strange and isolated individual . . . probably single, in his late twenties and living in the low-lit basement of his parents' house, smoking cigs. Now, of course, I know the Mena thing on the whole is certainly true.
Also, I might point out, I considered myself fully non-conspiracist despite the fact that I fully disbelieved the official JFK and RFK narratives. These somehow didn't count because they were so old, which was ridiculous on my part. My first exposure to 911 conspiracy I now see as an almost perfect example of cognitive dissonance meets conspiracy realism: I somehow, absurdly, pictured a missile hitting the Pentagon plane exactly as it was entering the Pentagon, thus solving the problem without having to revert to deep conspiracy. I mean, it never even crossed my mind, even as a staunch Bush hater, that it could be anything other than something like that . . . and yet the evidence was right in front of me and really only pointed one direction. Then, after being a fully confirmed believer in 911 conspiracy, I still didn't really believe any others for quite a while. I didn't want to! What person, who considers themselves an intelligent and careful thinker, wants to be known for believing something that looked wildly absurd to them only a year before? I'll answer my own question: no one. It's a bit painful, to be honest.