You want to die? Go ahead.
If my cat doesn't die naturally before she gets to a point where keeping her alive is cruel, I have already planned to have an at-home visit from the vet. If I can administer it myself, I will. She shouldn't be handled by some strange person she doesn't trust in her final moments. I don't want her to die scared. It's horrible for me to think about, but she is old and sick, and I don't want to get caught off guard when it's time to make that decision, so I have thought about it. I very nearly did it about a year ago, but fortunately a last-ditch medication effort brought her back to a good standard of living.
Why wouldn't I afford a human loved one the same consideration I give my cat?
No one should force the family to, of course. If they are unwilling, the person can be provided with a medical assistant to do it instead.
If they still want to, it's their life to end as they please. But giving them the opportunity to know those sorts of things is important.