The Vedic literature offers two answers.
First, we're being given a chance to live out our desires. You want to fly? Take the body of a bird. You want to swim? Take the body of a fish. You want to drink blood? The body of a tiger. Fool around and have sex all day? The body of a monkey.
Second: We're being given repeated opportunities to attain spiritual realization, break free from material entanglement, and resume our eternal nature in the spiritual world. The Vedic writings are meant to guide us in achieving this goal.