Conflict

Lajos Egri, in his classic, The Art of Dramatic Writing, calls conflict the "origin of action" and "that titanic atomic energy whereby one explosion creates a chain of explosions." But perhaps James M. Frey puts it best in How to Write a Damn Good Novel. He says that story is struggle and that conflict is the "gunpowder" of storytelling.

Conflict and struggle go hand-in-glove with each other. No conflict, no struggle. No struggle, no conflict.

Conflict not only moves the plot, it brings characters to life. For, in the light of conflict, character is defined. That's because the way people respond to obstacles, resistance, barriers, and conflict sharply characterizes them, telling us who they are. This is largely because conflict forces them to make decisions and act. So, when you bring your characters into conflict, you breathe a soul into them.

A stretch of action or dialog without conflict — present or imminent — is, in a word, boring.