Let's make it simple. Our mother is the one who conceived us in her womb. When we're already born, she's the one who provided us with our basic needs like food, shelter and clothing. In short, our mother is the main source of our needs, without her, we can't survive--without her, we are nothing. That is why nature is compared to our mother because just like our mother, nature is the only one that can give us everything we need. Without it, we can't live--we can't survive. The nature is given to us by God so that we can continue to live.