Origins of criticism in vanity. Birth of the critic in the undergraduate classroom, where truth is the podium on which pretentious youth elevates itself above the masses. Criticism as self-congratulation: the critic knows better, is the chosen one who will rid the world of error.
The noble mission of criticism. Maturity and immersion in the cause bring self-forgetfulness out of a genuine desire for a changed society. Passion for the world's potential supplants self-aggrandizement as the critic's motivation.
Self-corrosion. Love of what ought to be, gradually, becomes contempt of what is. The slowness of society to change disillusions the once idealist into a misanthrope, seeing only the world's worst. As acid eats its container, years of acrid words corrode the speaker's humanity.
Criticism seldom changes society for the better, but often changes the critic for the worse.