That's not a place I want to be.
Sure, my preference is immortality of some kind, but at some point choosing non existence seems reasonable. IMO, as soon as you decide everyone get's the right to get off the ride at some point then I think you must also leave that choice up to them.
Can a terminally ill child choose to die? What if they have a significant life long illness that isn't terminal?
What if they have no illness?
Not saying it is impossible to work out all of these problems, I've just never seen anyone do so.
There was a great article posted here on HN a while back by a number of highly-regarded psychologists that stated most suicides in men (who make up most suicides in raw numbers) are not actually due to mental illness at all, but rather cultural and philosophical factors combine with certain events in their lives leading to a sense of irreconcilable failure.
Hell, seppuku is a form of ritual suicide that has nothing at all to do with mental illness but it arrived at by a set of personal and moral values.
I'll see if I can dig the article up.
I also knew someone who suffered great pain on a daily basis, and I have a hard time considering his suicide irrational. I knew a guy (a friend of a friend) who, through foolishness in his youth, ruptured three of his spinal discs, and had damaged two other ones. He was in constant excruciating pain. In the trailing decade of his life, he could not get sufficient pain relief to hold a job, could not get/afford treatment because of that, and basically was in a pain-filled hell, as he put it. At some point, after he'd been in near-constant excruciating pain for twenty years, he decided he no longer wanted to live.
I don't think either of these men were mentally ill, or making irrational choices.
No man is an island, even if he feels like one sometimes.
Keep in mind that I'm making an ethical and moral argument. To a large degree, whether suicide should be legal is another discussion. To say someone shouldn't commit suicide isn't the same thing as saying it should be a crime to commit suicide.