Because it's an affront to their beliefs and values. Most people are raised to value life by there teachers and parents, and so when someone expresses the idea that maybe the human life we are taught to value Arbitrarily, isn't as Great as they have been told to believe, it seems as though you are attacking them, for there want to live. They feel A societal need to coerce you back to there way of thinking, and take it as a personal insult to them that you don't comply.
Society also raises us to be members of our community, they raise us to contribute to it, and so when you are perceived as not contributing by others, you are viewed as being led astray, and that it's societies duty to bring you back into the fold, even if it means violating your own wants and decisions. Thus they feel as though they have a right over you, and your life, and they chose to view you as being there own property, as though you're a child or pet.