Perhaps the most compelling reason it is difficult to define life is the lack of objective measuring tools. All of our human methods for defining the undefinable (science, philosophy, religion, metaphysics, etc.) are self-limiting in some way. Unlike other living organisms, human beings seem to be driven to quantify and categorize the world around them. If we can describe a phenomenon such as "life" well enough, we can bring some order out of chaos. The problem is, once one working definition has been created, a previously unknown plant or animal may appear and defy the definition.
Scientists have several qualifications they use to define life, including the ability to reproduce and a reaction to outside stimuli, such as light or heat. But certain computer viruses can use electronics to replicate themselves, and some inorganic materials can be engineered to respond to outside stimuli — plastics that shrink from exposure to heat, for example. Computer viruses and engineered plastics are not usually considered to be living organisms, but they each satisfy at least one of the criteria scientists use. Scientific methods and principles alone cannot adequately describe all of the elements of life.
Just as non-living objects can have similar qualities as living organisms, living organisms may have similar qualities as those objects that are not alive. For example, a human child contains a measurable amount of iron, sulfur, zinc, calcium, carbon, water and salt. Coincidentally, a random sampling of gravel and topsoil also contains these elements. Life is not, therefore, completely defined by a list of elemental ingredients.
Experience tells us that other elements found in the world (minerals, water, metals, etc.) may contribute to life, but are not filled with this undefinable force. Humans can quantify the objects around us as animal, vegetable or mineral, but we cannot capture and examine the life force that supports the largest tree and the smallest one-celled organism.
From a philosophical or metaphysical point of view, life occurs whether we humans can measure it or not. The fact that we are sentient (capable of self-awareness and thought) tells us that we are indeed filled with this force. Plants and animals that share some of our own organic structures are also said to be alive.
Religion has also played a role in our quest to define life. Many religions believe that life is a gift sent by a benevolent Creator who set in motion all of the biological processes needed to sustain this force. Power of this magnitude and scope is beyond human comprehension, so many people feel compelled for spiritual and philosophical reasons to accept the undefinable qualities of life.