Why does Thoreau believe "a government in which the majority rule in all cases cannot be based on justice, even as far as men understand it"?

Essentially, Thoreau
believes that a government based on majority rule will not be a just one because the majority is
simply the strongest group, not necessarily the right group. It is not fair to the minority,
whose opinion in any and all matters of legality is basically rendered completely irrelevant
even if the minority is in the moral right.

Thoreau argues that conscience
should dictate our decisions as individuals and that the citizen ought not to be required to
"resign his conscience to the legislator." He argues that we should be individuals
first and citizens second; our own consciences should rule our decisions, and we should not by
ruled by the laws, which are determined by the majority vote of our legislators, who are elected
by majority vote. Thoreau says,

Law never made men a whit
more just; and, by means of their respect for it, even the well-disposed are daily made the
agents of injustice.

Majority rule leads to a society in
which unjust laws are created, and these lead to unjust practices in society; for example,
slavery was legal once and we can all agree with Thoreau that slavery was unjust, barbaric, and
unconscionable.