Why did Ayn Rand describe rationality as the highest human virtue?

Ayn Rand identified rationality as the highest human virtue because she considered reason to be "man’s basic means of survival."1

In her philosophical system, Objectivism, Rand argued that life is the ultimate value for all organisms. For humans, this means living specifically as a "rational being." Because reason is the primary tool humans use to gain and keep values—and ultimately to preserve their own lives—the act of exercising that reason (rationality) is the fundamental virtue required to achieve one's goals and sustain existence.1

By prioritizing rationality, an individual pursues their own life and happiness, which Rand termed "rational selfishness." Conversely, she viewed altruism—the sacrifice of one's own interests for others—as a fundamental vice, as it negates the use of reason as the guide for one's own survival and flourishing.1