Aside from the morality question, it bothers people (who've been around long enough/have seen enough of how things happen) because they know that it's a slippery slope: first you can decide, then your family can decide, then "medical professionals" can decide, then it "somehow" becomes "policy". (Ex. an acquaintance in The Netherlands has said that, if there's a shortage of hospital beds, they are given to younger patients rather than older patients alledgedly because they have more life to live [are more valuable]. From there, it'll only be a short step for them to, "Who should be unalived/die [because they're not "worth the time/effort/resources"]?)