I think the main problem with throwing everyone that is suicidal into the mentally ill category is the implication that we can't make rational decisions for ourselves. (Maybe you could argue that our thinking has become negatively skewed- so we can't... I don't know.)
It would very much come into play if assisted suicide clinics are legalised across the world. 'No- we won't allow you to kill yourselves because you simply aren't thinking clearly. We need to protect you from yourselves. Here, have a Prozac and some therapy- it will make you feel better.'
Also, as it stands now- writting off every suicide as a result of mental illness tends to overshadow everything else. WHY were they mentally ill? (I guess I'm largely focussing on depression here which is perhaps one of the main 'illnesses' that seems to have the most variations in intensity.) So- If it was depression- why were they depressed? Did they experience trauma or abuse at some point? Was it financially motivated? There ARE reasons behind why a person takes their life other than- 'they're life was fine but they were crazy.'
Sometimes these other reasons paint an unpleasant picture of our society- some people here are suffering due to the negligence of larger companies or other people. I expect the majority of us are now struggling to pay the bills because our salaries don't match the cost of living. Some people are suffering in abusive relationships. Some people simply can't have or achieve anywhere near the life they want and can't stand their life as it is. It's easier to put the blame on the person who just killed themselves though.
How many times do we hear how 'well' these people were doing? With bright futures ahead of them- but they clearly didn't see it like that- were they wrong? Or is life just bloody difficult? Is it a mental illness to look at your current situation, look at your prospects and feel like the future REALLY doesn't look good? I think there's this notion that- well, everyone struggles- you just have to get on with it. But why? For those around you- ok. For society- a begrudging- ok. For you though? That really depends on whether you CAN get better...
I certainly agree that it isn't 'natural' for any animal (including humans) to want to kill itself. The primal part of our brain STILL does it's best to cling on to life despite what the thinking part wants. (Why we have to battle SI to do it.) So- is the thinking part 'natural'- I mean- of course it is- it's bilogical. But does it function in a 'natural' way? Perhaps not- it doesn't act off of instinct for a start. It takes in all the things our bodies experience and it ruminates on it. Why is it then considered ill when it sees the world as it is and thinks- this looks bad?
They can't really say having ANY negative thoughts is a mental illness although having predominantly negative thoughts seems to be. Again- I don't know- I'm not a scientist. Perhaps the brain does change, the more times you have these thoughts till you get stuck in a rut- low serotonin levels and all that. Still, do the MAJORITY of people you know just breeze through life or are they struggling too? We can't ALL be mentally ill! Or can we? Kind of scary...
Yet, if we say we're finding it difficult and it's causing us to be unhappy to the point where we want to take our own lives- we are classed as mentally ill. I mean- I don't know- perhaps if enough bad shit happens in your life, maybe it does change the way your brain works. Surely initially though- it was a basic apraisal of your situation- 'This sucks... Is it likely to get better for me?... Probably not- without an enormous amount of effort and maybe not even then. Is it really worth it then?' ...Why is that crazy? Seems kind of logical to me.
As for me, I wouldn't say I'm obviously mentally ill. I've just had my hair cut. I laughed and conversed with the hair dresser the entire time. I guess I'm 'lucky.' I still get to enjoy life here and there. Having said that, I am finalising my will and getting my method prepared. I'm waiting on my Dad to go first, so I don't know how long I need to stick around- I don't know I can even do it then... I am a complete coward.
My reason? My creative freelance career has financially failed. I don't want to go back into some shit job that I'm going to hate. My freetime simply won't be enough to distract me from how much I hate the job and hate my life. Of course- I could be entirely wrong- I could actually find a job I like but my 42 years of life on this planet suggests otherwise (for me.)
I want to quit because I have no interest in being part of the system anymore. I guess I could go on anti-depressants like some of my friends that seem to zombie-fy you so you can cope with your job and your life. (A shocking 1 in 3 people are on ant-depressants in their workplace!) But I don't want to. I don't have enough in my life other than my creative work to make it worthwhile.
Again- I COULD make the effort to try but I also know from experience that you can't really rely on friends. For me, it feels like a rational decision. Is there some mental illness there? Probably- I definitely think I'm obsessive, somewhat depressed and have social anxiety. Could my life improve- possibly but only with an ENORMOUS effort on my part.
Am I at a point where I'm unable to make rational decisions about my future? (Including ending it) No- I would say I have enough of my faculties about me to understand the choice and make a logical decision. Sorry for the long post.

and thank you if you got this far!