Wordsworth believed the the "world" (all that is not nature, eg. the city) corrupted the natural piety that we were born with. This is a key Romantic idea, and it is clearly illustrated in Mary Shelley's "Frankenstein." The creature is created and is full of love and kindness, searching for it in return. When he meets "society" (whether in the city or in the "society of man" in the countryside), he encounters hatred and fear.
Wordsworth remembers the joy he felt in nature when he was younger, and is thrilled that he can still experience this joy when he comes back to it ("Tinturn Abbey"). Whereas many men become "practical" and lose their sense of nature and what they can learn from it, he has not. If the goal of human life is to grow in the moral sense, then "the child is father to the man" because the child has this wonder as part of his nature, and, in time, gives "birth" to the man who has often grown apart of the child.
In another of Wordsworth poems:
"The world is too much with us, late and soon,
Getting and spending, we lay waste our powers.
Little we see in nature that is ours."
You might want to read the notes on "Tinturn Abbey" below for a clearer sense of Wordsworth relationship to nature.