Great post
My step-Dad is the same way as your Dad and I always try to cheer him up, except he is very stubborn.
On the other hand, I have a really good friend that is 83 (BTW I'm 43), and he's one of the most cheerful person you could meet. He's 83 and he has and had his share of health issues, but it doesn't bring him down, and it keeps him looking young. One of his favorite quotes is "Is it really that bad", and really if you look around, there are lots of people that are a worse off that us and don't complain.
I consider myself a very positive person, and mostly because of tragic events I've had to endure. That's why I also have the same attitude as your supermarket man.
We only have one life, and it's a pretty short one, so don't stress over the little things.