When I was a teen, about 14 or so, I attempted suicide. I ingested a bottle of aspirin, I remember counting out the pills, 96. Luckily, I vomited most of them up. I experience about 12 hours of agony (metabolic acidosis with respiratory alkalosis), but I survived. I felt so foolish afterward, I didnt even tell my parents what I had done.
Why did I do it, I was not sane. To this day, I know that the reasons that I used to justify my actions were rediculous. But at the time, they seemed real and overwhelming. I realize now that it was a moment of neurotransmitter deficency, that manifest as severe depression. There was no life trauma to blame.
I was soon back to normal mentally and emotionally, and time showed that my fears were baseless.
So the likelyhood that suicidal patients are not making rational decisions is very high, the medical community knows this and so society chooses to protect these patients.
The converse of this is the patient with chronic pain or terminal illness. These patients may be allowed to minimize there suffering even if it hastens their death. It may be hard to know when we should allow them to actual enact their own death, as each case is different.
Depression is a disease, a chemical disease, much like diabetes, one cannot chose not to have it. There are therapies both medicinal and behavioral.