As someone else brought up, there can be disagreements between family and an individual about when the right time for them to die is in both directions. Even in America this can be observed with things like life support for the very elderly, an intervention that is sometimes necessary for continued life but not considered euthanasia to forego. You won't have to dig very deep to find a doctor that has seen families argue about this.
So, if you say "only I can make the decision," then mental illness becomes a lot more deadly than it has to be. If you say, "other people can weigh in," then what do you do when they want you to die more than you do (and start pressuring you, and you're old, and already depressed...)? This leaves us at our current state, which is "nobody gets to make the decision."