The issue I have with suicidality automatically being construed to be a consequence of mental illness is that this argument always tends to assume the suicidal person is behaving irrationally and not in their right mind. Many people conflate mental suffering with being an unreliable narrator and incapable of making sensible decisions for whatever reason, as if everyone who is suicidal is in some pseudo-psychotic state of delusion with 0 lucidity or insight into reality.
Indeed, many types of psychotherapies like CBT are rooted in the foundation that mental pain stems from having incorrect and overly negative thoughts, and that if you tell yourself that your thoughts and feelings are irrational cognitive distortions enough times, you can correct this because it is a conscious decision to do so. I can't even begin to explain what an oversimplification this line of logic is, yet it forms the cornerstone of many "gold standard" mental health treatments since they are offshoots of behavioural psychology.
It is true that there are lots of people out there who decide on suicide impulsively, with an extremely sudden onset and no history of psychiatric conditions. In such cases, maybe you could say someone lacks emotional regulation or long-term decision making skills necessary in order to assess all their options before jumping to the most extreme conclusion. But there's no mental illness you are going to be diagnosed with those traits alone. There's certain criteria that has to be met to receive an on paper psychiatric diagnosis. The same thing happens whenever some violent crime happens, people just throw out the word mental illness meaninglessly and cast the blame on this unspecific concept which in turn leads to more distrust and castigation of your average depressed or bipolar person who is non-violent, but both are going to be thrown under this wide umbrella of being mentally ill- when we can't even define what that even means as a collective.
There is so much we don't understand about the brain and I think it is simply easier to default to a vague, nebulous explanation of all suicidal people being mentally ill than trying to explore the variety of reasons why someone would want to die in the first place. Mental illness is often used as a catchall and unspecific term to encapsulate every type of mental suffering whenever it is convenient. There is a whole lot of fear surrounding the topic and very little understanding.
I think I have strong feelings towards this subject because the entire concept of mental illness has been used to invalidate so much of the pain I've been through. On top of my physical health problems, I have PTSD and autism and have been involved with the mental health system since a very young age. As soon as your problem gets labeled as mental, there is so much judgement and unscientific assumptions that you "choose to be this way" and somehow have control over it. Your credibility goes to the dogs and everyone acts like you know nothing about yourself and your own life.
I have taken so many medications and spent years doing therapy, and when their methods didn't work, they will always claim without fail that it's because you don't want to get better or try hard enough. It took many years for a doctor to tell me that the treatments out there simply weren't adequate for my level of complex PTSD, but then others will continue this nonsense that it's mental, you aren't cooperating when medications don't work, or a therapist makes you feel worse, and even worse, you're hurting people on purpose by not getting better.
Here's the thing, I told myself many times it isn't "rational" for me to have PTSD and the responses my body is having to innocuous stimuli don't make any sense. This did not make one lick of difference for me. I'm still traumatized, I'm still suicidal over it, I still have these responses, because my brain developed around shit that happened decades ago. There's no official treatments for PTSD except psychological/talking therapies and the same SSRIs that get thrown at everything, there is very little science in this field being implemented in practice.
There's complex biochemical responses going on here that guilt tripping and shaming simply do not fix. And yet I have still been treated like an absolute piece of shit who willingly sabotages myself and doesn't want to get better simply for admitting these tactics don't work, they're harming me, and saying honestly I need something different! One time I argued with paramedics and police saying that the healthcare system was not helping me or taking my physical health problems seriously, and that the GP was just invalidating my issues and my PTSD as well. They deadass gave me a blank eyed stare and kept telling me to talk to the GP and that help is out there but I have to "want it or seek it" or something like that.
It simply makes no sense whatsoever to me to keep telling someone who has tried dozens and dozens of different treatment methods that they're just stubborn, not seeing reality, "unwell and troubled" and every other pejorative in the book. Let's face it, no matter how much these campaigns talk about destigmatizing mental illness, the reality is that when a problem is too complicated for modern healthcare to solve they slap you with the label of mental illness/psychological issues and act like it's your fault and you have some kind of divine control over it. The healthcare system at its core stigmatizes mental pain that is long term. I think many suicidal people in the early stages of things could see improvement if others acknowledged their pain rather than treating them like they're irrational and crazy for feeling this way.
There are so many reasons why people choose suicide, it's extremely complicated and can't be reduced to a single factor. When dealing with chronic illness, one of the most invalidating things I have been told over and over again is that it's not having disabling physical conditions that's the problem, it's that I'm depressed over it, and I should just have a more positive mindset, that it's my mindset that is the issue and not the horrible symptoms causing me to feel bad in the first place. If we were all irrational and unable to see reality, I don't think the suicide rate would be so high in patient populations of conditions that don't have a treatment or for those whose chronic pain is unmanaged.
I think people are really ignorant of the consequences of long term health conditions and suffering, as well as how quality of life impacts a person's will to live. It is far easier to call everything messy, complicated, and uncomfortable a mental illness than examine what it means to be mentally well, sane, logical etc in the first place or to develop new treatment options to help suffering people who get thrown these diagnoses.