I think part of our problem as a society is perceiving painful mental conditions as somehow less devastating/more treatable than an equivalent physical condition (I'm not saying you believe this, just using this as a jumping off point). There are physical conditions and diseases that we tend to accept as being detrimental to worthwhile quality of life, as a matter of course. Our understanding of similarly crippling/life-ruining mental issues falls way behind our understanding of the physical ailments we'd put into that category without much thought. We think the mind is more easily fixed than the body, which obviously isn't always true.
This isn't anyone's fault--it's harder to quantify something we can't see or more likely, can't fully imagine or empathize with. People used to a normal state of mind also tend to believe that having a higher level of control over their own thoughts and feelings is the natural state of things, and it isn't for everyone. We think applying willpower and "sucking it up" can get us through mental and emotional turmoil--and for most of us it can. For most of us our mind is not our own worst enemy.
I don't know what was up with this 17 year old, and I'm extremely open to the idea/argument that 17 is too young to know for sure it wasn't fixable. But I do think there are people out there whose minds are so utterly ####ed in ways I can't comprehend, and that science cannot right now heal, as to make merely existing an endless trial of living through hell on earth, day after day. For these people, like with some late-stage, terminal cancer patients, sometimes the merciful release of death might be the most humane treatment option.
The tricky part is that we trust someone stuck in a physical hell to be mentally sound enough to trust their judgement when they want to let go and die. Because their mind isn't sick, their body is. Meanwhile, we don't necessarily trust someone's judgement when it's their mind causing them issues in the first place, maybe understandably. And that's a minefield to navigate, idk how we take the next step on this or reach that next level of understanding.