Here's why:

* Viscosity is the internal friction within a fluid that resists its flow. Imagine honey versus water. Honey has a higher viscosity, meaning it resists flow more than water.

* This resistance arises from the cohesive forces between fluid molecules. The more tightly bound these molecules are, the more difficult it is for the fluid to move.

Other forces may oppose motion in specific situations, but viscosity is the fundamental force:

* Friction: This force arises from the interaction between the fluid and a solid surface. It's important in cases like a boat moving through water.

* Pressure: This force acts perpendicular to the surface of the fluid, and can oppose motion in some cases, like when a fluid flows through a narrowing pipe.

Let me know if you'd like to delve into any of these forces in more detail!