[Faceman]

As cynical as it may sound, laws are not made for individuals, but for society as a whole.
Killing or hurting yourself does not only harm you and the ones close to you, but also loses society a viable asset.
That's (getting more cynical here) the primary reason why many people would be opposed to assisted suicide if performed on healthy person.
If the person is "lost" anyway we are far more inclined to allow him/her to "end it".