Anosmia - the loss of the sense of smell, either total or partial.
That is what I have. I don't know why.
It happened around age 8.
It began as partial but is now almost total.
For the most part I don't even think about it.
Around Christmas, I am more aware of it.
One of my favorite books when I was little was
The Sweet Smell of Christmas
by Patricia Scarry.
Every other page has a scratch-n-sniff sticker
associated with some wonderful Christmas smell:
I remember sticking my nose on each page taking it all in.
Last Winter they reissued the book.
I bought one!
I can't smell the stickers, nor can I remember the smells.
My brain has forgotten.
It is the only time it makes me a wee bit sad.