Is life a gift with a transcendent purpose to be fulfilled, or do we own ourselves and have the right to do with our bodies whatever we please? This question can be answered in part with a little reflection. Why do we feel compelled to talk someone out of suicide? Why try to dissuade them? The reason is that we have an intuitive sense that life has transcendent purpose. We're so sure of this that we try to stop people from killing themselves and "wasting" their lives.
A life can only be wasted if it has a purpose that is never fulfilled. If there is no purpose in life, there is no tragedy when a child is struck down in infancy, or when high school students are killed in a plane crash, or when 39 people commit suicide to fulfill a cultic hope.
Notice that the notion of "untimely death" here has no relation to the person's own subjective goals. The goal of a suicidal person is to die, a purpose he fulfills if he takes his life. An infant who dies unexpectedly has no goals or aspirations of his own. Yet in both cases we have this nagging suspicion that something is wrong.
Our sense of tragedy lies in our conviction that these people did not fulfill some larger purpose in life, one bigger than their own temporal wants and desires. If such a purpose exists--and our intuitions suggest that it does--then it isn't the case that our lives are our own to do with as we please.
God has made it clear that we are not the masters of our own lives. Our existence is not a thing we own, but a sacred life we are entrusted with. The commandment "Thou shall not murder" forbids us to take an innocent human life. It applies to taking our own lives and not just lives of others.
There's a reason for this. The fifth commandment was given not because murder violates personal liberty by taking something that belongs to someone, his life. That's covered in the seventh commandment, "Thou shalt not steal." Instead, it prohibits the unwarranted destruction of a human being because he's made in the image of God (see Genesis 9:6). Murder is primarily a crime against God.
Our lives are not our own, they’re a gift with a purpose to be fulfilled.