Why does diffusion of water happen when there are dissolved particles on one side of a membrane but not on the other?

To achieve Equilibrium of solute/solvent concentrations

Explanation:

Diffusion of water, known as osmosis, occurs to maintain equilibrium. Your body constantly wants to be in a balanced state also known as homeostasis or equilibrium and it achieves this using many different mechanisms. For example, your cells have Potassium Sodium pumps that are responsible for transporting sodium and potassium ions across the cell membrane. When there is more sodium inside the cell than outside, they cell pumps sodium out. If there is more potassium outside than inside the cell pumps potassium in all to maintain an equilibrium.