Memory and happiness

    I've had this topic at the top of my Notepad for months. I can't get it clear in my mind, perhaps obviously. The idea though is in line with the truism that older people are happier because bad memories recede. Well maybe we aren't happier but we do tend to go on about the good old days, and tend to forget about the bad old ones. I'm wondering if this is in any way connected to Alzheimer's and I'm wondering if there's any evolutionary pressure to have happy old people around.
    I guess it would be good for the species to have elders who are considered wise, and it might be easier to consider us wise if we're comparatively happy-go-lucky. Moreover, we might be more useful if we're able to share good things from the past that might be applied in the present. Of course, in the current political climate, it would be helpful if some old people would point out that the Trumps of the world are trotting out the same arguments that led to ships full of Jewish refugees being turned away and sent back to the Holocaust. So there are limits to the argument.
    What I'm mainly wondering is whether Alzheimer's or other forms of senile dementia are just an extension of natural memory loss. And then there's the question whether the kind of memory loss I'm talking about in elders (i.e., focusing on the happy ones) is a physical problem or a decision. Maybe as you get older, you just decide to define yourself as happy and thus to filter out the unhappy memories. I don't know. My memory, like the rest of me, doesn't have a lot in the way of filters. Gradually the unhappy memories have less sharp teeth. But they're still there, they're still there.