Bryan Caplan asks a question as to why people think he's certain when he's not.
First, I have the same problem, as noted in the comments section back in August 2009.
Second, because I've had the problem for perhaps 10 years (before then I really was that certain), I think it's pretty simple to explain.
If you are a smart person (3Sigma+), you get used to being the smartest person in the room, and soon you start acting like you think you're the smartest person in the room. It's especially tricky when you actually have heard all the arguments the opposition has to offer, and you think all of them are weak [a moderately common situation when one likes arguing, and holds a substantially minority position]. Not very long after winning an argument or 8 with authority figures who are supposed to know more than oneself, one picks up a bit of cockiness, and proceeds to think that some folks fecal material don't stink.
If one remains very good at what one does, and one has a different-than-most perspective, and one is a near-teen it becomes very hard to not walk into an argument believing that (a) one knows what the other side has to say, and (b) one already has a response. This comes off as cocky and certain. And then cocky and certain-sounding become habits.
When one ages a bit, recognizes that one doesn't always know what the other side is going to say, and that there are multiple angles to the question, and that one is mistaken at least moderately often. One's actual certainty drops substantially. But the habits, built over 10 or 20 years of experience, don't drop so substantially. Conversation patterns, once used in debate to provide emphasis, or effective counterarguments before an audience now carry an unintended signal of certainty. That, incidentally, still has positive effects.
I empathize with Bryan's problem...and hope I can eventually write with as much humility as Scott Sumner, Arnold Kling, or Robin Hanson...or else find a niche (not currently occupied by blowhards) wherein my natural style of communicating with certainty is an advantage. Say...teaching.