In theory, I'm in favour, but it's not an easy law.
My feeling is that it should only be offered right at the end - the person is on end of life care and will die within a week. Euthanasia only to hasten the imminent inevitable.
This introduces a different point though - I DON'T think the patient should be the one to make the decision, because by the time you are suffering and near death, you can't. And why should the person who has not considered whether they want assisted dying or not but found themselves close to death and in pain be forced to suffer?
A lot of people seem to be saying they'd want to die if they were diagnosed with dementia. But I do not think it should be used for this. First of all, by the time you are diagnosed it is debatable whether you have capacity for such a major decision. And secondly, it is possible to live well with dementia for a while after diagnosis. This length of time can vary greatly between people.
With the pet example, some people have their pet put to sleep because of the care and expense it would require from them. Or because of the impact of the pet on their life , for example a dog that bites. People will choose or encourage their relatives to choose (even subconsciously) to die for their own benefit if the criteria aren't extremely tight.
I am a carer and Mum had dementia. She is a burden and as time goes on she will become more of one. Caring for her takes a toll on me. That doesn't mean I don't love her. However, her death will be a relief and a release for me, which I cannot deny. How could I ever agree knowing that I benefit from the decision? She cannot make that decision for herself and, if she had made that decision before diagnosis, when would it be carried out and who would decide?
I don't want dementia either, but euthanasia isn't the answer to it, finding a treatment and prevention is. If euthanasia was legalised for dementia, finding a cure becomes less important sadly.