For the first question, I believe it is because prosecutors and actual murderers could argue that "the victim told me, the victim consented" and it would be a huge gray issue and gets complicated very quickly when it comes to consent and agreement. Furthermore, it is possible for the victim to change his/her mind, which could then be argued that he/she didn't really consent, etc. That's just one of the reasons why people are just outright against assisted suicide (in addition to all the other reasons listed). Another possibility is that if the person is drunk, under the influence of a drug, or coerced, and of course, many masses like to use the line of he/she is being irrational, not thinking straight, temporary feelings, etc. which I mean that could be true but again, it's the grayness of each situation that brings such moral dilemmas in people's eyes.
I personally would be in full support of voluntary euthanasia with systems and measures put into place to prevent abuse or any grayness. The problem is that most of society is still pro-life and anti-choice when it comes to anything related to 'death', especially suicide.
For your second question, yes I think there is some innate feeling in almost all humans who feel 'responsible' for another's death, even if the consenting person wants to go through with it. Perhaps it is just a part of being a human being, instinctual empathy and selfishness mixed in one. Very few people in this world are ever logical and rational and most people tend to lump in emotion and some passive investment into even a stranger, especially if a stranger is no longer around. This is partly why there are just doctors and medical professionals who are staunchly opposed to anything related to reliving suffering, death with dignity, peaceful exit, etc., because they don't want to deal with the fallout or have to face the reality that they took part in ending one's suffering.
I believe they are selfish, but sadly, that's what a lot of medical professionals think in the field.