Humans are self-organizing networks which have selected for valuing life. Today we use medical technology to ensure slow death, but human decay so challenges the rush for life that we have moved death to hide in institutions or in hospices.
Some self-organizing systems become repetitive. A few continue to iterate and change. Human development has selected for components which are drawn to the next and the new and to more. Along the way our genes have selected, over and over, for the survival of an adaptation. The prototypes that don't strongly value survival don't survive. At the same time human consciousness wrapped itself around the value of survival and more-ness and added stories and myths to ensure that more life was the dominant adaptation. So we end up with encoding for life and a story that human life is sacred. And we ennoble suffering, promising rewards after.
Some other reality might produce some other value system – perhaps the spiritual consciousness grew and 'mind meld' was the global value .
But here, now, life is considered sacred because we have selected for genes that are drawn to life and wrapped that process in a story. It's very fundamental and very circular and no reason not to choose to die a good autonomous death.