Because allowing it creates a culture where it becomes expanded to more stuff and then it becomes an expectation. When it's a cultural expectation it's no longer a choice one makes for themselves. It's a choice that society makes for you and anyone who tries to defy that choice faces the wrath of not only society but loved ones.
Laws change culture.
Divorce used to be seen as shameful. . . then we made it easy. Now divorce is not only normal half of marriages are ending in divorce. (Although to be fair a small majority of first marriages make it.). I
The acceptability of Marijuana use is also a case where law has changed culture. Now I actually don't pass any judgement on that as I think it should be legal and a place where a cultural change was probably beneficial. However the point is that once a few states started changing their laws the attitudes towards it shifted.
In the places where euthanasia was legalized the attitudes towards it shifted to where it's ok to hold down a dementia patient to kill her and it's ok to euthanize a physically healthy 29 year old woman with depression. Given that those were test cases they are likely more common now.
Belgium for example kills about 40 people a year for strictly mental illness related reasons.
Only two countries permit euthanasia in cases of mental illness — and they show how hard it can be to balance personal freedom and adequate protections.
www.statnews.com
What's crazy is if you read that article a person in Belgium can be euthanized for having mild autism. I do not want to see a world where my sons, one of which their autism is more severe are allowed to ask a doctor to kill them because of their autism.
Even worse I don't want to see a world were society pressures them to do that. I already have enough to worry about with their future including potentially their long term care situation for after we are gone. I don't need to be worrying about a country that in 40 to 50 years will be telling them that the morally right thing to do is to let a doctor kill them.
As far as the link about the woman being held down.
Panel finds the doctor acted 'in good faith' in controversial case
www.independent.co.uk
Prosecutors argued that a dementia patient euthanised in 2016 had not been asked to verify her wish.
www.bbc.com