Here's why:

* Life requires constant work: Living things need energy to carry out essential functions like growth, reproduction, movement, and maintaining their internal environment (homeostasis).

* Energy powers processes: Energy is used to build and repair cells, transport substances, and create chemical reactions necessary for life.

* Sources vary: Organisms obtain energy in different ways. Plants get it from sunlight through photosynthesis. Animals get it by consuming other organisms.

While other factors like water, oxygen, and nutrients are crucial, energy is the fundamental requirement for all living things to exist and function.