Question

why is viscous force is dissipative

Answer 1

A measure of a fluid's resistance to flow. Viscous forces in a fluid are proportional to the rate at which the fluid velocity is changing in space; the proportionality constant is the viscosity.
Viscous Forces Here the frictional force increases as the first power of the relative speed between the surfaces and opposes the relative motion. Viscous friction is important for wet surfaces at small relative velocties.
Viscosity is not a force, it is a measure of resistance of movement between molecules to each other. It is shear stress in the fluid essentially. If they cannot move respective to each other then they would consist of a solid.