The speaker’s state of being “recline[d]” and observing in solitude is fundamentally important to his philosophical realization because it establishes the necessary conditions for deep, undisturbed contemplation and facilitates the spiritual connection that allows him to discern universal truth.

Solitude Enables Introspection and Truth
The act of being “reclined” in a secluded grove is not merely relaxing; it is the prerequisite for the speaker’s emotional and intellectual journey.

Undisturbed Contemplation: Solitude removes the distractions and noise of human society—the very thing the speaker comes to lament. This physical isolation allows for introspection, enabling the “pleasant thoughts” to evolve and “Bring sad thoughts to the mind.” The initial sweetness is a product of peace, which leads directly to the pain of realization.

Accessing the Soul’s Link: The philosophical breakthrough—the feeling that “Nature link[ed] / The human soul that through me ran”—requires the speaker to be internally quiet and receptive. In the absence of other people, the speaker can feel this essential, spiritual bond, allowing the uncorrupted voice of Nature to speak to his deepest self.
Observation Provides the Moral Standard
The act of observing the natural world in such detail provides the concrete evidence necessary to form the speaker’s moral critique of humanity.

Detailed Evidence of Joy: The speaker’s reclining posture allows for quiet, sustained attention to small, intricate details: the “primrose tufts,” the way the periwinkle “trailed its wreaths,” and the “least motion” of the birds. This close observation builds a compelling case that joy and pleasure are the innate conditions of natural existence.

The Contrast: By closely observing Nature’s harmony (the “blended notes,” the “holy plan”), the speaker establishes a moral standard of perfection. This standard is then held against the abstract concept of human behavior, leading directly to the lament: “Have I not reason to lament / What man has made of man?” The observation provides the facts; solitude provides the space for the emotional judgment.
In short, the speaker’s solitude creates the receptivity needed for spiritual truth, and his observation provides the evidence needed to justify his profound moral sorrow.