Here's why:

* Composition: Water makes up about 70% of the mass of most living organisms.

* Essential Functions: Water is crucial for:

* Solvent: It dissolves many substances, allowing for chemical reactions and transportation.

* Temperature Regulation: It helps moderate temperature changes.

* Transport: It carries nutrients and waste products.

* Lubrication: It acts as a lubricant in joints.

While other compounds like proteins, carbohydrates, and lipids are also important, water's sheer abundance and vital roles make it the most abundant and essential compound in living systems.