This makes sense to some extent (terminal illness or such), but one can argue as someone did above that if you're willing to make the permanent decision to "destroy" yourself, are you sane enough to legally make decision? Are you
for sure not under temporary delusion, distress, or other threat? You can always "legally" "destroy" yourself through suicide, but if you want to involve a medical professional it seems very reasonable that you have the mental clarity to make the decision.
Should it be illegal for the doctor to deny me an amputation that I ask for? After all, I should be "free to destroy myself" absent any justification?