I see a fundamental difference between physical and mental illness at least in 2017: When someone with a terminal cancer or other terminal degenerative condition, doctors can state with virtually absolute scientific certainty that the person will die, identify the precise biological reasons they will die, map out the process and what sort of pain to expect, and even estimate when they will die with increasing accuracy.

Mental illness, however, is still extremely poorly understood and there are people who go through phases of extreme mental suffering, sometimes even for years, and who then recover and find contentment and happiness. To this day "depression" is an illness defined purely by symptoms and not by underlying causes. 

With respect to those in the throes of extreme mental suffering, there are many who come out of it. It would be a shame to have state-approved killing-with-consent (or "assisted death" as activists now call it) of those people. I am against that.