What is the driving force that causes matter to change phase?

A change in temperature.

As temperature increases a solid will melt into a liquid. A liquid will evaporate into a gas. As the temperater decreases a gas condenses into a liquid. A liquid freezes to solid. Sometimes the liquid phase is skipped altogether.

The temperature at which phase transitions occur vary for different chemicals due to different chemical structures and bonding. Also some compounds will decompose, rather than melt.