Yes you have to be a Canadian
Many years ago, you only require a permanent resident certificate to apply. However, you need to be a sworn-in Canadian citizen in order to apply.
To be eligible to become a Canadian citizen, you must meet all conditions, including:
Permanent Resident status
Time you have lived in Canada
Income tax filing
How well you know Canada
If you have served in or with the Canadian Armed Forces, you might be able to apply through a fast-track process. See website: http://www.cic.gc.ca/english/citizenship/become-eligibility.asp
Yes! I am one, but I aged out of the Forces.
Yes, this is to serve Canada so it should be for Canadian Citizens only.
Yes, you have to be a Canadian citizen or fall under a minority recognized by the Canadian Government.