TLC.....lol....I have, so far, written over 12 thousand responses, here on SR.
During that time, I have responded to many persons with your same question....
And, because I have my own intimate knowledge of this kind of situation..I have, so many times, written long, detailed explanations, in answer. Out my whole heart into it. Direct answer to direct question. I always felt like giving a concrete logical and (sometimes scientific) answer would, somehow, soothe and satisfy the person who was asking your same question.
Guess what? I have been wrong, for so long. Because, at the end...the person still says..."That makes sense...but I still think of him and I still miss him!"
I think that the truth is...no answer will satisfy. Because what you are feeling is normal, natural and expected at this juncture.
Think of all of the poems and songs written about this, over the course of history. Such a common human experience.
Now, if you had asked..."What can I do to get past this?" or " How can I get over this?".....I could give a more satisfying answer...at least, what worked for me and most other people. But, you didn't ask that....You asked only one question..."Why does my brain still think of him."
Actually, when I step back and look at it...I think that for people who ask this question---it is really, mostly, rhetorical....The need to ventilate and share about what they are feeling (which, of course, is o.k.)....That a concrete, factual answer is not really what they are looking for......
thus, I won't bore you with one of my half page answers....I will just say that it hasn't been long enough time....it is along process....
******Please note that I began writing my post before your response to HardLessons.....