You do realize that there do exist such situations where animals would have been put to sleep instantly but strangely humans are left alive, right?
Say, you have an accident. You wake up, lying in a bed, incapable of doing anything. You can't talk and there are all sorts of tubes stuck in your body. A doctor appears and sees you are awake and tells you that you are a vegetable and there is no hope for recovery and that all your close relatives died in the accident. Day in day out, you see the same parts of your room. You can't converse with the world around as everything is out of touch. How can you not want to pull the plug?
OR
Say, you are a sporter. Doing sports is your whole life. Being able to move makes you feel like you can take on the world, so wonderful as it is. Then, by reason unown, you move your arms and legs, despite having lived an extremely healthy life. No more doing the things that 'kept you alive', no more feeling the rush of being able to move, you completely lost the capacity to without any hope of recovery. You are jealous of all your sport friends that still can walk and you find it utterly glaring and painful that you can't compared to them and all the memories of moving suddenly have become excruciatingly painful. It is for you, unacceptable and therefor you beg to end it but no one replies.