I didn't think you were joking, I was just wondering if the question was relating to yourself or perhaps someone else.
I have absolutely no idea man, surely it would depend on the history of the patient. If there is a history of suicidal ideation/attempts there may be a question of that person's mental health. Although the intent of someone signing such a form could just be a statement of that person's beliefs in that when their time comes, that's the end. In which case there probably wouldn't be much of a question of their mental state.
Having said all of that I am certain that any doctor would largely question someone's motives for signing such a form.
Regarding the DNR issue - it is a common thing that comes up when someone is trying to get their affairs in order. I worked as an estate planning assistant to lawyers for many years - primarily for the elderly but we did have young clients that were not ill - and as part of their estate plan, nearly all requested living wills. Most of them were of what we called the "pull the plug" variety and designate someone to act on the person's behalf if they were incapacitated to the point they could not make their own medical decisions.
We did this in 2 separate documents; one of which was a "Designation of Health Care Surrogate" and the other "Living Will". In my experience, most people do not wish to be kept alive by artificial means if there is no realistic medical hope for recovery. I have both, naming my father and my aunt, and they are part of my medical chart. My version also includes a provision that I be administered pain-relieving drugs even if they hasten my death, but only if my doctor rules that I would be left without any hope for recovery/declared brain-dead.
It is a morbid thought, but the thought of remaining on machines as basically a vegetable is awful to me. My family agrees with my wishes and have made the same provisions for themselves.
The first lawyer I worked for gave the clients an extra copy specifically to be placed in their medical chart. I'm not sure how it works in Aus or in Canada where kaywholed lives. It may be called something different. I do know that my own doctor did not freak out when I gave him his copy. My surrogates each have one as well (as do I as to them as I am their designated surrogate).
Kaywholed, I'd advise you to check the laws in your jurisdiction and if necessary, consult an estate planning attorney. Plenty of people do it online or with purchased form kits, but the only way to make it airtight is to talk to an experienced lawyer, IMO. If you are having suicidal thoughts, talk to your doctor right away. These documents should only be signed when you are of sound mind and can make the best decision for yourself.
I hope this is of some help.