I've witnessed my father with dementia and a whole lot of other diseases (kidney failure, MS). He was
happier in his last days^H^H^H^Hyears. Except for pain because of his back from lying down, an indirect result from his lower leg amputation.
(Somewhat related to the lucidity: he had good and bad days, and I never knew which one it was going to be before I went seeing him. This lead to both surprises and disappointments.
I also suspect my father used a lot of humor in his life as a way to memorize things. Because humor sticks. Now I remember all these dad jokes, and repeat the pattern with my daughter...)
Now, contrast to my mother in law. I'm currently the only person who speaks to her. Her 3 children are in fight with her, due to various reasons (well, I believe it all stems from autism which 2 of her 3 children now have a diagnosis for 1 of which she does not even know; and she surely has it as well, same for her oldest son). Her memory and brains are still quite well at the age of 77 (6 years older than my father when he passed away).
I don't have such a good memory, and it haunts me currently. It has haunted me all my life. Especially other people with autism have much, much better memory than I have.
I believe dementia has a rather clear purpose: to become happier in the last days of your life, to make life bearable.
Since I learned about this purpose, it makes it easier for me to know I have this gene. And more difficult to know how my partner is going to deal with the lack of dementia plus the occurrence of mine.
From my observations between my mother with my father, and myself with my father, it seems at first an internal struggle which the patient is trying to carefully hide. Eventually they admit it to other people. It seems that at some tipping point, the pain of dementia is more for the people who notice it, while the person with dementia does not suffer as much anymore.