According to this article 79 year old retired academic Lisette Nigot killed herself in her own home and left a suicide note claiming that she was inspired by euthanasia campaigner Philip Nitschke...he beleives that there is such a thing as 'rational suicide'. i don't know anything about this case, but i don't believe that no sane person would want to kill herself or himself because i consider myself sane and i can imagine scenarios in which i wouldn't want to live. and i don't just mean fantastic nightmares such as a world where all musicians sound like the backstreet boys. i don't think i'd want to live if i had no ability to communicate with other people, or if i lost my ability to learn. i don't think that makes me insane. but i do think there are cases in which people should be stopped from killing themselves.
and i don't have an answer to the obvious question of who or what guidelines should determine when a suicide should be allowed and when it should be prevented. but suicide itself provides some initial basis for making such decisions, by distinguishing between those who are and are not intellectually and emotionally able to actually carry out their own suicides. it's almost like a reverse natural selection. if you're fit enough to carry out a suicide, there's a good chance you are doing so reasonably. if you can't manage it for some reason, there's a good chance you shouldn't be making important decisions for yourself.