Why does Thoreau refer to civil disobedience not merely as a right but as a duty?

Thoreau refused to pay an extra tax for the war in Mexico and went to jail for it. He believed that government had to have the consent of the people to wage war or collect taxes. Thoreau believed that the liberty of the individual transcended that of the state. He felt that it was the individual's duty to oppose "the machine" when it oversteps its bounds,

"Let every man make known what kind of government would command his respect, and that will be one step toward obtaining it."