Something tells me that your dad is probably not suffering from a mental illness..., right?
No. With all due respect, but the problem with people like your dad is that while he supports the right to die for physical cases (which is good) he, and many others, only goes by what he can see, what he himself has experienced and perhaps worst of all is downplaying mental illness as a whole. When he says people just don't want to get better and could be easily cured in essence he's thinking "they're having a bad patch and just don't want to snap out of it". This is so wrong.
I've mentioned many times here that my life was supposedly great until I got these issues at 16. Yes, I really did love life and the memories kept me afloat for a long time after. Some here may thus assume I'm a freak, who was high on dopamine all the time. Not so. I wasn't maniacally laughing when one of our au pairs was trying to charge up the stairs attempting to bludgeon my great aunt to death with an iron bar and all that stood between them was my mum blocking the way, with me behind her, protecting us all. Must've been 7. There certainly were negative events, but they never broke me. These are things your father, amongst others, simply doesn't get. It's not necessarily everyday "outside" issues/events which affect one (although they too can often be the origin, think trauma, rape etc.), but mental illness goes straight to the core, shattering one's entire being. It's much more serious and vastly different from having a "temporary" bad patch which one could possibly get out of. Which is why so much advice and assertions are ultimately futile.
A lot of good arguments have been shelled out already, what I agreed with most though, apart from the obvious general right to die, was getting tired of trying time and time again, evaluating the illness and generalisations. Mental illness today is everything and nothing. All painted with the same brush. When you're a hammer everything is a nail. How many people here, or everywhere, do you think have had a CAT scan, as in actual visual determination and thereby evidence of the physical manifestation of their illness? Hardly any surely, and even if they did, that doesn't mean the "professionals" actually know how to deal with it. The brain is poorly understood and popping random medications is just scratching the surface for many. One of the main and most famous proponents of SSRIs admitted a few years ago that they don't actually know how they work, just that they do. What he failed to mention was "for some" or "temporarily". Well, when I was healthy I didn't need medications, nor was I having severe side effects from being so. But it's logical that SSRI and similar do, because in essence they don't understand the whole process, as in (physical) origin of the illness and medicational effects on the brain. Therefore, for a lot of people there simply is no cure.
Do agree with your father on thing though, have indeed given up - finally, after 25 years (now 30). The first seven were the worst and the last five a complete dismantling of my entire self. An event in 2018 made me lose an essential luster for life and question my whole existence. Basically, why am I still doing this to myself? Pain and suffering, physical and mental, for little to no joy. It's just not worth it anymore. Have had four psychiatric evaluations in my life. Some diagnosed things which were indeed fitting, but none of them ever essentially told me the same thing! Depression was the first, of course, followed by anxiety, social phobia and agoraphobia. Those are merely symptoms though. Also display autistic and even some schizophrenia simplex symptoms. My mind is not functioning properly, not like it used to. Personally, I really think it's physical brain damage.
Which is why the last psychiatrist in early 2019 probably hit the mark best when she said I should get a neuropsychiatric evaluation. She literally said to me that "she doesn't know what it is, but there is something", and in her report she wrote it was impossible to tell if I'd ever get better again. Finally, someone was humble enough and honest. But it's too late now. Am going on 50 and my health is in ruins, the seen can't be unseen and don't really like this world anymore, it has become a lot worse compared to the time I was happy. Want to get out, and doing it right and comfortable is essentially my last drive.