My dad used to be a very upbeat, positive person until he got into his 80's. I talk to him about looking at the glass half full and to focus on all the good things. But everyday it is the same thing. His negativity is very draining to all around him. His health has improved 10 fold over the past 6 months. However, everyday he says he is getting worse. He is getting upset with me as I try to turn the negativity into positives. Perhaps I need to be more empathetic but then I feel I am feeding into his negativity. Just would like him to be happy but maybe I am asking too much and should just accept this as the norm.
Do a little digging and see if you can't discover what may be ailing your dad. Find out why he says he's getting worse everyday. Maybe there's something going on with him.
It could just be age related decline. I can see that a person who is aging would not be as upbeat as they were years earlier. Try to see things from his point of view.