Suffering animals getting humane euthenasia = compassionate. Humans seeking euthanasia=cruel practice. Why?

Why do some people think of humane euthenasia in animals as a compassionate thing to do yet are strongly against human euthenasia a person themselves choosing to end their suffering on their terms.

Suffering animals getting humane euthenasia = compassionate. Humans seeking euthanasia=cruel practice. Why?
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