First, know that these mood changes are the direct result of the disease process (rather than just an emotional reaction to having the disease).
Try hard not to take apathy personally. You may bear the brunt of a blunted mood, but the reaction isn’t to you any more than to having dementia.
Avoid pointing it out and getting on your loved one’s case. (“Why aren’t you happier? It’s such a beautiful day.”)
Better: Be empathetic. Avoid anger.
Continue to seek out activities that once brought pleasure (a visit from grandchildren, a drive in the country, watching sports, listening to music) and engage your loved one even if you don’t get the same response you once did.