Why should you care about the loss of biodiversity--the mass extinction taking place right now--caused by our neverending expansion on and using up of the planet?
Why should people care about the fate of these plants and animals?
In the most simple terms, we should care because the quality of our lives ultimately depends on them.
Without species diversity, we wouldn't have the healthy ecosystems that supply our food, cleanse our air and water, provide sources of life-saving medicines and help stabilise our climate.
We would also miss out on a free and ubiquitous source of miraculous beauty and endless possibilities.

From a viewpoint article on BBC, here.