This post is not going to invoke the deeper philosophical question about what is natural and what is unnatural. Instead, it is going to critique the categorization of suicide as unnatural death under the accepted and commonly used definitions of natural and unnatural.
What is natural death? Roughly speaking, it is when one or more systems of the body fails to perform its' functions to an extent that is required for sustenance of life. One can die of 'natural' causes when heart or liver or kidneys give up. Sometimes more than one system fails leading to multi organ failure. Why is the brain discriminated against then? If someone's brain gives up due to depression and therefore takes his or her own life, how did it suddenly become unnatural? If heart gives up, then the body does not receive blood thereby cannot get oxygen resulting in death. If brain gives up, it uses it's signaling mechanism to signal to hands to tie a rope around the neck and tighten it (the example of hanging is used here but the idea any can extended to any form of suicide). This results in cutting off the supply of oxygen resulting in death. I do not see much difference between a heart giving up and a brain giving up. I am against this discrimination against brain. Brain is like any other organ and its failure is no more unnatural than failure of other organs.
P S - This argument can be extended even to death due to accidents. I think accidents are a 'natural' part of our existence and so should be categorized as natural death. I can accept homicide as unnatural. One can argue that the ill intentions of another person resulting in one's death is also natural but that is at a philosophical level which I don't intend to consider for this blog post.