Why do we consider euthanization of other animals humane, but of humans morally wrong?

It’s not a new topic, but one that’s bothered me for a while.

How shooting a lame horse in the head, or giving a shot to a sick dog, etc, is all considered the kind, “merciful” thing to do. That they are better off dead than living in misery.—Not that we can really communicate with them, in the sense of asking them if they’re really miserable enough to die, if they really want to be put down. We’re assuming that we know better than them, that they can’t understand the situation; we percieve them living in a life of what we see as unbearable pain, and a life that we have the right and responsibility to end for them.

But when people feel miserable and want to be able to go, whether on their own or with help, we consider it wrong and an irrational act of desperation. Even though the person is able to tell us that they want to, something that the other animals we decide we should kill can’t.

Why such a double standard? Why do we get to decide to take other animal’s lives from them, but not our own from ourself?