Not exactly a lurker, but I seem to go in spells of posting. In "lurker mode", folks have already covered most of the reasons (oh boy, coasting on blank -- hear ya Nammu!), but another comes to mind. I often read to keep up with what's going on with everyone while I'm on the bus. Responses generally take me longer to put together than I have to write them. So, time. But often often often it is just not being able to think -- like you say, glamslam, "staring at the screen".

Then other times ya can't shut me up, lol!