Because I would be participating in killing a living human. It's our duty to avoid intentionally doing that, even if it really hurts.
Technically it's the same reason that I don't want someone to euthanize me right now. The value of my life does not derive from feeling happy or otherwise using my senses. Nothing important changes when I have brain damage (or cancer, or depression). All I have to do is be human, and my life has a certain type of value that makes it wrong for anyone to intentionally end it.
If I try to build in loopholes, or start messing with semantics, or if I compromise because something is going to hurt, I end up being unable to act from reasoned moral principles, which is something else I value.
(I have a similar curiosity, if you'd oblige, about what makes euthanasia any different from simple suicide - I actually asked that earlier, but all I got was downvotes.)
Technically it's the same reason that I don't want someone to euthanize me right now. The value of my life does not derive from feeling happy or otherwise using my senses. Nothing important changes when I have brain damage (or cancer, or depression). All I have to do is be human, and my life has a certain type of value that makes it wrong for anyone to intentionally end it.
If I try to build in loopholes, or start messing with semantics, or if I compromise because something is going to hurt, I end up being unable to act from reasoned moral principles, which is something else I value.
(I have a similar curiosity, if you'd oblige, about what makes euthanasia any different from simple suicide - I actually asked that earlier, but all I got was downvotes.)