>Somehow the idea that someone can be perfectly sane and not want to live as long as possible is inconceivable to people.
There are many people who have considered suicide, chosen not to follow through (or failed), and then realized that they were in an emotional state of mind. Suicide is a generally pathological phenomenon - horrible things will happen if we don't accept this as the null hypotheses in specific cases.
And yes, plenty of people have setup advanced directives, DnR's, and other such instructions while fully on their capacities, and that doesn't require them being in a pathological or even emotional state. It can easily be caused by having a standard of quality of life they intend to maintain.
Edit:
I feel compelled to add a qualification to what I've posted.
If anyone thinks they are rational and suicidal: I don't believe you. Self-harm is an emotional behavior. Our brains are designed to help us lie to ourselves when we're emotional. The odds that you are the one philosopher in an arbitrarily large crowd of people who's needs aren't being met is incredibly slim.
I grant no one abstract approval for ending their own life.
The idea that there is a biological 'switch' underlying depression is misleading. It is in a number of cases but it's only a small percentage.
The crux with depression (and mental health more generally) is that it's a vastly complex phenomenon [1] that we do not fully understand.
We are scoring wins in many battles (e.g. MDMA-assisted therapy) but we are still far away from winning the war.
The implications are pretty troubling to me personally. Someone gets diagnosed with a sketchy mental condition, which in turn causes them to contemplate death, and then the very same doctors approve their euthanasia.
If I tell anybody to kill themselves I will rightfully get criminally charged. Someone approving an euthanasia is pretty close. Sure, they may want it, but so will people who get denied, one cannot say the act of approving is completely meaningless.
You're confusing not wanting prolong life in a state of unbearable pain because of an incurable, terminal illness, and willful, intentional suicide or homicide. There is a very big difference.
In medicine, a distinction is made between ordinary and extraordinary care. Ordinary care concerns the basic needs of the patient such as air, food, water. Extraordinary care concerns medical treatments meant to cure or otherwise treat the disease or disorder. Typically, neither a patient nor anyone else can deprive a terminally ill patient of ordinary care. However, when providing of such things constitutes an "undue burden", such as when the body is no longer able to digest food or absorb liquids and the offering of food and drink actually causes harm, the patient may refuse food. At that point, you are accepting your powerlessness in the current situation, i.e., one in which the patient is in a state from which he cannot recover and for whom death is imminent.
Even treatments that may extend the life of the patient can be licitly refused by the patient if it would, again, pose an undue burden (for example, a weak, terminally ill patient may refuse CPR if his condition is such that the procedure, which is physically traumatic, while potentially able to extend the life of the patient by some small amount of time, would leave the patient in a state of great suffering during their last few hours).
So extension of life at any cost is not the goal of most people who oppose euthanasia. Natural law theory is perhaps the most vehemently opposed to euthanasia and will serve as an excellent resource for considering these kinds of questions.
I would caution you against making appeals to subjective feelings in a vacuum, which your examples attempt to do. This is dangerous because we are capable of feeling all sorts of things and being confused about all sorts of things. The crude subjectivism of our times puts excess emphasis on such things without understanding them in an objective context (which itself may be misunderstood or crudely understood).
In a hypotetical future, it is possible assisted suicide rates will be down just before elections and up after elections because a group of people have already decided on the suicide, and are delaying the procedure to be able to vote.
They should really get over it. Just like those with different gender preferences they won't ever understand they will have to just take people's word that it's sincere and they're doing it of sound mind.
The rationalizations I hear people use are really hollow to me, like someone claiming a gay man just hasn't met the right woman yet. It's fundamentally different.
All of my friends who have taken their own lives struggled with it for years. I wish they weren't gone but I have to respect if after all therapy and help options were exhausted, they still were committed, then so be it. I wish they were here right now, of course I do. It fucking sucks.
It's personally traumatizing but as my final act of friendship I was there to support them in their decisions.
I had to sign off the release forms for my friend's ashes 6 years ago and drive up to santa barbara to give them to his mother. I remember sitting there in silence at her kitchen table with her when she said "we all knew it was the right thing."