✔ 最佳答案
It is probably because of people's instinctive aversion to dying.
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I agree that each person should have a right to guide own life and his death (to the degree that they can.)
In the U.S. each person is allowed to choose any career he wishes and can even choose "no" career. It is legal for him to live anywhere, drive anywhere and buy anything according to his desires and abilities.
Every person must die. There are no exceptions. It should be made socially acceptable for an adult to end his own life for any reason regardless of health. (He can do that anyway, no matter which law is broken and no one can do anything about it after the fact. A great many people do that.)
Why is it considered a good thing to put a suffering animal to death for "its own good" but not allow people to do that with themselves?