My mind is constantly in motion and often is engaged in multiple activities at once. Part of it is just the speed at which it works. In college, I would bring a crossword to some classes that seemed to go just a bit too slow. I would continue to follow the lecture, but when the professor went on a little too long or belabored a point, I'd fill in a couple of clues to keep from drifting off in boredom.
The problem with this is that when something needs my full attention, it is difficult. My brain wants to work on some design or something in the background while I attend to the task at hand. I had a friend call with some rather tough news that needed my complete attention. I went off by myself, turned off the radio, and sat in my recliner as he recounted what he was going through. It was tough to keep myself focused on understanding the tough times he was going through and not start plugging away at a Sodoku or something.
But the other problem with this conversation, was that after it was over, my brain kept working in the background on this conversation. A little later, there was a list of questions in my head that I wished I had asked, but it was (IMO) inappropriate to start up the conversation again just for those questions.
Even posting here, once I hit the publish post button, give me a couple hours and there will invariably be something else I wish I had said. For example, when I was posted about going to the chiropractor (now months ago), I wanted to make the witty comparison between my opinion of them and the doctor in The Pearl.