Why is that most aging parents are so stubborn when their children urge them to change their unhealthy habits?

I noticed that this was the case with my grandparents when i was younger, and my parents were persuading them to see the doctor, change their unhealthy eating habits, exercise, etc. And now it's the same for my parents! I have to urge my dad to take fish oil, eat less fatty foods, exercise, and eat more veggies! Is this is some instinctive behavior of humans that when we get to a certain age, we have the urge to keep unhealthy living habits, and to not listen to our off springs? Or that their live styles they have kept since forever have nostalgic effects on them? Or that... maybe they don't want to live longer?