Here's why:
* The theme of transience: The poem centers on the inevitability of change and the fleeting nature of beauty. The "gold" represents something precious, but its temporary nature creates a sense of sadness.
* The imagery of spring: The poem uses the imagery of springtime, a time of renewal and growth, but also highlights the swiftness of its passing. This creates a bittersweet feeling.
* The final line: "So Eden sank to grief, / So dawn goes down to day." This line emphasizes the inevitable fall from grace and the loss of innocence. This reinforces the melancholy mood.
Overall, the poem acknowledges the beauty of life's fleeting moments, but it does so with a sense of resigned acceptance. There's a tinge of sadness in the understanding that even the most beautiful things must eventually fade away.