The universe is a place that, all other things being equal, tends towards equilibrium, or balance. In other words, molecules and atoms will not spontaneously organize themselves into towers against the pull of gravity – but trees (among other examples) will. Molecules and atoms will not spontaneously organize themselves into highly concentrated solutions of sugars and salts in small droplets of water – but single-celled organisms in a pond will.

Without life, the tendency in nature would be for the towers of trees to fall – as happens when they die. The tendency of solutions inside cells is to disperse – as happens when they die. It is the metabolic processes of the living thing that prevents trees from falling or cells from dissolving. Also, without their towers, or highly concentrated solutions, these organisms cannot reproduce, since reproduction requires particularly delicate chemical processes to all go right in order to work. The organism uses metabolism to create a protected environment for its reproductive chemistry to go forward.

Metabolism is what changes the environment of an organism to allow that organism to live and reproduce. No metabolism, no life!