Why did Heidegger begin Being and Time with an analysis of Dasein?

Martin Heidegger began his seminal work, Being and Time, with a phenomenological analysis of Dasein because he considered it a necessary prerequisite to properly address the Seinsfrage, or the fundamental question of the nature of Being.1

Heidegger rejected the traditional Cartesian view of the human individual as a "thinking subject" or a mind isolated from the material world. Instead, he introduced Dasein—meaning "being there"—to characterize the human individual as "being-in-the-world." By replacing concepts like "consciousness" or "mind" with Dasein, Heidegger aimed to show that human existence is the locus where entities disclose themselves and where Being is uncovered.1

Because Dasein is the entity for whom its own Being is an issue, analyzing its structure—including its active character and anticipatory temporality—provides the essential access point for understanding the nature of Being itself.1