My Oma went through Alzheimer's. Sbe went from being a functional adult, to forgetting how to speak English, to mistaking family members, to recognizing no one, to being unable to feed herself over the course of 10 years. But she seemed happy most of the time, usually smiling, would light up when she saw people, she would attempt to sing along to whatever music was playing. So I'd say she remained happy most of the time, which made it much easier to deal with from our end.