It's quite simple in my opinion: people who want to die will just commit suicide if they don't get help, so why not offer them a comfortable way? That doesn't only make it easier on the dying person but also on the family (I can speak from experience), and it saves someone from the trauma of finding the body and the costs of a police investigation and cleanup.
I don't think there should be an over the counter pill you keep in your nightstand in case anything hurts you though. It seems appropriate to at least require filing a request to your home doctor, followed by two or three sessions with a psychologist. If someone then still feels dying is the only solution, gather the family around and let the person go comfortably.
