There's a seasonal metaphor that thematically unifies walden, although you only read the "spring" essay. Why do you think Thoreau used a seasonal metaphor?

Thoreau employs the repeated metaphor of rebirth throughout his book, as a means of convincing his readers to seek new perspective on themselves and the world. The cycle of the seasons, with the rebirth of the winter-dormant pond, animals, and plants in the spring, functions as the promise of an eventual spiritual rebirth in humans. Likewise, Thoreau's description of the hunter boy who grows to be a naturalist as a man and his metaphor of awakening from the slumber of life evince his hope and belief in the progress of human beings to a newer, greater understanding of themselves. He ends the book with a final metaphor of rebirth, describing the bug which hatched out of a wooden table after decades, in the hope that some day, even if not immediately such a rebirth will occur within human society.