By personifying Nature as a mighty but affectionate deity, the poem suggests that the natural world can be a guide, a teacher, and a parent—and that human beings might hope to model themselves on nature's example.
At the beginning of the poem, Nature waits around for a minute to watch little Lucy grow from birth to three years old, sprouting up like a flower through "sun and shower." That's all the time it needs to decide that "she shall be mine, and I shall make / A Lady of my own." Here, Nature becomes an artist, molding Lucy in its own image:
"Myself will to my darling be
Both law and impulse: [...]
In other words, Nature makes Lucy into a little copy of itself, giving her both its "law" (its structure and rules) and its creative energy. In fact, Nature's whole personality seems to be about such balances. Being Nature-like, Nature reveals, means having a perfect blend of energy and calm, playfulness and seriousness, joy and wisdom.
Being Nature-like also involves being able to find beauty in troubled places: the wild “motion of the Storm” itself, Nature declares, will only give Lucy more grace. Nature, then, is unflappable. Even the most dangerous energy fits into Nature's grand scheme.
If Nature is loving, wise, and balanced, it's also stern. Its "law" is a law of death as well as life. When Lucy dies, it's as if Nature is completing its work on her by fully absorbing her, literally making her part of itself (an idea that Wordsworth returns to in another of the Lucy poems, "A slumber did my spirit seal").