The speaker in this poem is very pensive about the idea of the passing of time. I believe Stevens uses leaves to present such thoughts because leaves change color, which is a display of their life cycle. In the beginning of their life the leaves are a vibrant green, and before long the season changes and they turn a red - orange color, and once they lose strength they fall to the ground where they turn brown and dry out. The wind then comes along and blow the dried out leaves away from the tree. And the cycle repeats over and over.