I wonder how these people live with themselves, and whether the people who "assist" with the "assisted suicide" really know whether they can handle the psychological effects of participation.
I liken your questions to what anti-hunters ask me. Few of them have grown up in the country, and even fewer have seen what happens in those areas that don't allow hunting. On the other hand, I've seen whitetail deer starving, half-ripped apart or badly weakened.
Perhaps if I hadn't known things, I couldn't have lived with myself after shooting that deer last week. But I did know that, and I also realize that life is decisions. Not assisting a suicide doesn't make everything better--it just means the person has no option to choose what they want for their life.
From being close to the right-to-die movement, I've known people who have chosen to take their life under their own terms, rather than spend their last days in decline and resentment. I think that actually knowing such people, and realizing the issues that arise in such cases, would make many people support assisted-suicide initiatives.
Perhaps that's why many people who are trying to take away peoples' right of self-determination often try to portray "assisters" as monsters, demonizing them rather than recognizing that these people are often torn up inside, and only their compassion for the patient is what allows them to do the right thing. Our selfish motivations make us want to keep a person alive for our own needs, while it's the patients who have to deal with the downsides: pain, discomfort, and lack of control over their life paths.
I imagine that if I were ever in a situation of actively assisting a suicide, I would try to broaden my perspective beyond my own desires--but that doesn't mean it would be easy.
But then again, isn't that what loving someone is all about -- wanting what's best for them, fulfilling their desires, not just being greedy?