I've a lady with Alzheimer's disease, let's call her Gladys.
She's living on her own but has frequent support from family who live nearby (very near, it's about 2 to 3 minutes walk from their house to hers) so has folk around her much of the time.
Still, they have their own lives so are busy and aren't with Gladys all the time.
Much of the time Gladys is low in mood. She's down, feeling miserable and glum. She doesn't cry, but feels as if she could. She doesn't see much of a future for herself, with nothing to look forward to. She eats what family cook or bring, but doesn't enjoy food, has no interest in it and no appetite. She has no guilt, no initial insomnia, no early morning wakening, no broken sleep, no significant weight loss, no diurnal variation in mood. She's never self harmed and has never had suicidal thoughts or feelings or impulses.
Gladys has tried different antidepressants. She's tried them at the maximum doses, so she's tried them at a therapeutic dose. She's tried them for a minimum of 6 weeks, so she's tried them for a therapeutic duration. She's no better on them. She has no evidence of a chemical mood disorder (endogenous depression) so it's no surprise that fiddling with the chemistry (i.e. taking tablets) has made no difference.
Still, it's hardly a cheery scenario. Alzheimer's disease damages your mood before it damages memory (with changes in the limbic system being the first changes in Alzheimer's disease) so she's structural changes in her brain that mean her mood's likely to be different. Changing or low mood is likely, then.
She sees herself becoming a burden to her family (they already invest huge amounts of time caring for her rather than just being with her), they won't entertain social services home care since they believe they can do it better and are happy to do so.
Gladys is miserable because she has a terminal condition that's getting worse by the month, robbing her of her memory and speech and personality then will kill her.
Unlike cancer, there's no hope of surgery, no hope of chemotherapy, it's destined to march on and take its toll.
She's low in mood because she should be.
Not an easy scenario to manage.