Until laws are passed nationally in most places it is illegal to have assisted suicide. Only the individual can decide what is best for them.
As of now physician-assisted suicide in the United States is legal in the states of Oregon, Vermont, and Washington. Some ill folks have relocated for that very reason.
Many pet owners have euthanized their pets rather than have them endure the often painful experience of dying. It's humane.
Deep down I believe more people are in favor of allowing the terminally ill to transition on their own terms.
What worries most people are those who might take their lives because they flunked out of college, their boyfriend/girlfriend dumped them, they were fired or laid off from a job, or it's been raining for two weeks straight! Depression plays a large role in most suicides.
It's one thing if a person decides to take their own life because they couldn't handle the ups and downs of life but very few people would support "assisted suicide" in those instances.
Lastly there is the matter of some family members having a financial incentive to (hurry along the death process) for their "loved ones" against their will.
That's why it's important to spell out what one wants in the event of...etc.