Pathos

The 1963 march on Washington was the biggest rally of the Civil Rights Movement. Martin Luther King, Jr. knew he couldn't accomplish his goals without getting everybody possible involved—African Americans, white people, politicians, students, adults, etc. This is the job of a leader: to inspire. And, often, the best way to inspire people is by playing to emotion.

Like a preacher, MLK aims for an emotional spirituality. One of his favorite tools for this is metaphor: "Now is the time to rise from the dark and desolate valley of segregation" (6.4). The imagery here references the idea of "the valley of death," a motif in Christianity.

The most famous line of the speech plays to emotion by making a plea for children.

I have a dream that my four little children will one day live in a nation where they will not be judged by the color of their skin but by the content of their character. (15.1)

The line places responsibility on activists, not to bring about change only for themselves, but so their children can have a better life than them. That's another facet of the American Dream. Nothing gets folks' tears running like references to their kids, their grandmas, or their cars.

Note that this speech doesn't include much specific evidence, statistics, or even anecdotes about racism. Playing to emotion, the speech allows the audience to fill in the blanks with stuff they already know about: segregation, Jim Crow laws, injustice, police brutality, etc. Referencing these sensitive points gets the audience emotionally involved, without dragging it out for too long.

MLK also gets into some fist-pumping rhetoric, repeating the word "now" (6.1-6) to describe the urgency of the African American situation. Nothing gets adrenaline going like a sense of urgency. The "I have a dream" section of "I Have a Dream" is also a tearjerker.