Why Are Human Bodies Creating Morphine?

An article by National Geographic tells of how the human body has been found to create morphine http://news.nationalgeographic.com/news/2010/04/100426-natural-morphine-drugs-humans-mice/ . While scientist found how the body makes morphine little is know about why. One theory suggests that poppies make morphine as a defense mechanisms against predators. However while this theory works for poppies and other animals such as mice, for me it does not work very well for humans , for very rarely are we hunted. While creating morphine may help us block out pain during an attack why would the body constantly make it? The mystery of why our bodies create morphine may never be answered. Since it has been shown that our bodies are constantly creating morphine it must essentially to our lives in some way that we do not yet understand.