I wish I knew what things mean to HIM...

A few days ago, I was talking to the Doctor while he looked over his to-do list (mostly mumbling, 'I didn't do that... Didn't do that either... or that...'), and then he suddenly said, "Why have I written down 'Clear throat'?" I went into a fairly uncontrollable fit of giggles as he murmured to himself how strange it was, then he loudly cleared his throat and said, "Well, that's done. I can mark it off now." It definitely didn't help my giggles, and, falling back onto his bed laughing my head off, I saw him actually marking it done! As I was calming down, he said, "It was in inverted commas... I still don't know what I meant..." It was all very strange. Then yesterday I asked him how staring at a screen all day at work doesn't give him eyestrain. He mumbled "Staring at a screen all day... Staring at a screen all day..." to himself a couple of times and then started singing about it! I can't remember what he sang, but it was very odd and fairly tuneless. To be fair to him, he wasn't singing very loudly, either. He went on for quite a while, and I gave up on getting an answer and got more interested in wondering what he was doing. Finally he stopped, and, after a long pause, absent-mindedly said, "Do I? Oh, yes, I do. Something must be done about that..." Sometimes I wish I could understand what he's thinking. He does so many things that, to me, seems extremely strange, but they make sense to him. I'm sure it would be very interesting to know how, because it might have to do with all kinds of things in his life!