We put animals "down" why not people?

I don't really agree with euthanasia for the most part but if the animal is old & going through an extremely painful experience that will last the remainder of their live (like some cancers), it would be the write thing (in most cases). It seems like people put their pets "down" for the stupidest reason sometimes too. So if a person is in a similar situation, I don't see why we couldn't (upon their request) put them "down" like with the animals, sometimes it would be the right thing to do, but we draw it out and make them suffer, just wanna see how some people respond to get a better idea of peoples opinion on this.