I can see some cases for person wanting a DNR to be obeyed, like if they are already dying from something that will be inevetible and painful.
However, consider this:
When Abbott and Costello were put in front of a firing squad, Costello was asked if he had one last wish. He siad. "I want to die of old age".

It's up to each person, to define what old age is, and the quality left in their life.
Without getting into religion. a physical death is permanet, so what's the rush?
I personally want to live as long as possible. As long as the day is more good than bad, in terms of pain, and is still a quality lifestyle but that's me.
__________________
"It doesn't cost "nuttin", to be nice". 
MOM
I just want to do the right thing! Uncle Joe, (my hero).