Sometimes something occurs to you which is worrying, and you start to think about it, and then get distracted from the thought - by someone talking to you say, or the phone ringing, or whatever. When the distraction goes away, you are left with a feeling of unease, like there's something you should be worrying about, if only you could remember what it was. Sometimes, in this state, you find something else to worry about, not the original concern, but something which you feel the need to sort out by thinking about it.

It seems to me that a version of this low-level disquiet is actually most people's experience most of the time, that they are either distracted from it, or trying to sort out some other, secondary, worry. The original problem, the something that occurred to them that they've forgotten, and have been trying to get a thinking grip on ever since, is, in my personal opinion, being thrust into a separate body in a hostile universe. In other words, between birth and whatever age it was that we learnt to forget about it, living hell happened, repeatedly, and it has been nagging at us ever since.

Next time, therefore, someone asks you what's on your mind, I suggest you breezily respond, "oh, nothing, just the profoundly insecure experience of being mortal, alone and in enemy territory."