Perfection is graciously dealing with imperfection. - David Allen

That means as soon as you tell yourself that you might need to do something, and store it only in your head, there’s a part of you that thinks you should be doing that something all the time. Everything you’ve told yourself you ought to do, it thinks you should be doing right now. Frankly, as soon as you have two things to do stored only in your mind, you’ve generated personal failure, because you can’t do them both at the same time. This produces a pervasive stress factor whose source can’t be pinpointed.

GTD essence: attention cleared of residue & distraction, pointed at the right thing.

Most often, the reason something is "on your mind" is that you want it to be different than it currently is, and yet:
• you haven't clarified exactly what the intended outcomes is;
• you haven't decided what the very next physical action step is;
• and/or you haven't put reminders of the outcome and the action required in a system you trust.
That's why it's on your mind.