Metal_Man wrote:
Better to be thought a fool than to open one's mouth and remove all doubt.
That's a lot of it with me. Once I start talking, my urge to "inform" can take over the whole show, and I begin to pour out everything I know that's in any way related to the subject (and a good deal that isn't, just for good measure).

So I often just keep quiet instead.
But there are also other things. There's the "start-stop" problem....once I've been quiet in company for a little while, somehow I find it physically more difficult to begin.
Also I've been trained to stay quiet. I was brought up in the days when the old "children should be seen and not heard" thing was still around, which made for an orderly environment but also got me into the habit of feeling that to speak was in some way wrong. To this day I'm much happier to answer a question than to just come out with information that hasn't been asked for, as if I interpret the prompt as permission to speak. In other words, I feel safer if I just speak when I'm spoken to, which is another nasty old expectation that used to get laid onto kids. There ought to be re-training sessions for people who have been repressed like that.
Things get a lot better when I feel I'm among friends, though that can take a long time. Also if I really put my mind to it and plan in advance to speak up at a particular event, I might break through the programming, though I can easily be thrown if I sense there's anything wrong with the conditions.