We accept that some physical conditions can't be cured, correct? They may be able to be managed to a greater or lesser extent but if you have one of these conditions you will always have it and will never be cured.
Let's say you have one of these conditions and despite your best attempts to manage it, you can't. You live every day in agonising pain. People may or may not try to help but seeing you in such pain is upsetting for them. Would you really judge a person for wanting out of that life?
Yet for some reason people won't accept the same for some mental health conditions, that some people live every day with agonising pain of depression, cripplingly low self-esteem, loneliness, isolation, flashbacks to abuse. For some reason, where MH is concerned, there's always a "cure" and I have been here long enough to have seen anyone who ventures to say anything about depression being sternly told to Get Help. If only it were that easy! And there is no guarantee of the "help" working, even if someone does seek it.
I'm not depressed but quite frankly no one would miss me if I died, not really, and if I ever go back to where I was, mental health wise, I will make a judgement on what is best for me.
Oh and people don't throw themselves under trains to say "fuck you" to the world, they do it because they know it will work.