Though the idea isn't something I'm fond of myself, to me this falls under a personal decision that shouldn't be prohibited by the government.
That's why living wills are so important.
The argument of prolonging suffering is subjective; because we're not
the patient, it might not be prudent to assume we know their level of
suffering (or indeed, what constitutes suffering) - but more importantly, we might not know what they would
want us to do.
Xpert asked about mentally and physically disabled patients. That's a different question, really - you're talking about ending the life of
someone who would otherwise live for an extended, or even normal period...that's different from ending the life of a terminally ill patient.
As Brian said, it often falls down to two reasons - either a religious belief, or a familial "need" to keep the patient around. In the case of the
former, the only religious belief that should matter is that of the patient - if my family decides that because they're devout, I shouldn't be
allowed to end my own suffering, that'd be a far more heinous moral crime than the actual administration of "more morphine than is necessary" to
hasten my death.
Sadly many family members do wish to prolong the lives of their terminally ill relative because they themselves
cannot (or do not wish to) deal
with the patient actually dying. Nobody wants to go through the loss of a loved one, and that's understandable.
Because I've rambled a lot (there's a surprise, huh?), I'm going to summarise my own stance:
Euthanasia should be an option if certain criteria are satisfied (the patient is able to make the decision, ie is of sound mind). You can't force a
patient to accept medical treatment - why should it be ok to force them to live in pain?
I have a couple of weblinks for those interested - I'm trying to update the list to include more nations:
US - living will information
Australia - living will information
Canada - living will information
UK - living will information