Much of it comes from Christian theology.

Suicide has long been considered one of, if not the, worst possible sins in Christianity. At least in the Catholic tradition, sins can be forgiven by confessing your sins to a priest and having them absolved. But you can’t do this with suicide. Per Christian theology, even a murderer or child molester can some day repent, beg forgiveness, and be forgiven of their sins. They won’t be absolved from the earthly consequences of their actions, but they’ll be forgiven in the next life. That is a core message of Christianity - no actions are truly irredeemable as long as you still draw breath.

But with suicide, this isn’t possible. You can’t confess your sins after you’re dead, and suicide means that your last act on Earth will be a mortal sin. I suppose you could maybe do confession along with assisted suicide. Maybe you have a priest on hand, swallow the poison, and then immediately confess your sin. But most religious scholars would likely argue that doesn’t work. Your contrition has to be genuine for it to count.

Anyway, pardon the digression. But this really is the root of it. Even in modern Western societies. Even among people who aren’t themselves active Christians. Even among those who’ve never stepped inside a church. Secular Western society is still heavily influenced by Christian philosophy. A strong aversion to suicide in any form is a part of this. For most Christians, voluntarily signing up for euthanasia is the easiest direct path to eternal damnation that one can achieve. The only quicker more direct way would be a murder-suicide. We’ve never had that same worry with animals. Christian theology doesn’t assign souls to animals. And even if it did, they would have no moral blame for us choosing to put them down.

At least in the States, I believe it’s for religious and financial reasons. Correct me if I’m wrong, but allowing someone to off themselves could be condemning them to hell. Also, to be cynical, medically assisted “checking out” is the easier, cheaper way out, instead of burning through money in a hospital.

Personally, I don’t see anything wrong with ending the suffering of a terminal illness. Prolonged suffering is unnecessary, and a person should have the right to go out on their own terms.

Religions and doctors “vowing to protect life.” Especially religious doctors “vowing to protect life” even when the life means just pain and suffering that can’t be properly eased with pain meds either, because you know, the dying person might get addicted to the meds. That’s obviously worse.

In my country, when an elder person is too sick and “ready to be euthanized”, they just stop giving them water and let them dry to death. It can take weeks. They do give some pain medication, but there is no way of knowing what amount is enough. You’d imagine that dying that way is pretty damn painful yet they don’t have a way of communicating that. But if they OD’ed, it would be murder so better let them suffer!

But also, euthanizing animals is becoming more taboo too. Many pets live in pain, relying in “pet mobility carts” and medications. Antidepressants for cats, epilepsy meds for dogs… Vets prolong the suffering for money, for people who can’t accept facts and do the kind and right thing. Animals have no way of communicating about side-effects from medications. Endless rehoming is thought to be better than letting go.