He almost couldn't help but write about Nature - it was in his very being right from the start.Nature was almost part of William Wordsworth's very being, even as a baby. His early childhood home was in an area of outstanding natural beauty in England - a mountainous area called the Lake District. In fact the bubbling of the beautiful river ,the Derwent, almost sang to him as it ran past the home. Later in life he recorded how its 'steady cadence' has instilled a deep love of peace, beauty, calm and nature in him and it's little wonder this expressed itself in poetry. Later in life whilst at university in Cambridge, he spent a beautiful vacation touring France, Europe and Switzerland on foot. His deep love for nature was something he shared with his sister Dorothy and the two were often to be seen walking, or admiring hosts of daffodils. Later in life he would also record the less positive side of nature, such as it's menace and brooding oppressive threats such as treacherous mountains and fogs. (The Prelude.) It was almost as if his awareness of the powers of nature helped him to form some form of communion with spirituality... as if Nature is in some way, is more powerful than man himself.
WHEN Contemplation, like the night-calm felt
Through earth and sky, spreads widely, and sends deep
Into the soul its tranquillising power,
Even then I sometimes grieve for thee, O Man,
This is probably because William Wordsworth was a Romantic poet (in fact, most say he was THE Romantic poet...the one who really got the movement going). Romanticism, in its simplest definition, was the movement where writers and artists tried to unite the natural world with the mind/emotions/soul of man. For instance, in what is arguably his most famous poem, "Daffodils", Wordsworth probably wanted to actually put his reader in that field of daffodils, and he was successful because all of the descriptive detail and sensory images he used: "Ten thousand saw I at a glance,/Tossing their heads in sprightly dance."
You may want to read more about the history of the Romantic period. I'm certain you can find several great websites on this on the Internet, but for the quickie version, try Wikipedia. I've attached the link for you.
Just to elaborate upon the answer given, you can just have a look at the different ways in which Wordsworth and his contemporary romantics were portraying nature. The philosophical sources are diverse. There is a good deal of German transcendentalist connection here with influences of Kant and Schiller. Rosseau's dictum "Return to Nature" is yet another impetus. There is a Spinozian element too. Let us look at some of the ideas Wordsworth and fellow Romantics were trying to communicate through nature--
1. Nature as in purity, innocence and man's relation to nature at the level of sensations...his growing experience as in Tintern Abbey
2. Nature in its inward form--human nature--the affective space within. All this was a reaction against the Neo-Classical fixity with urbanity.
3. The politics of nature...Man's manhandling of nature, the movement away from nature critiqued as a metro-centric neglect of rural life in all its simplicity.
4. Nature as a way of approaching God...the mystical experience of it...Spinoza's pantheistic notions of Natura Naturata and Natura Naturans come in over here..e.g. The Prelude
5. Nature as an impetus/stimulus for poetic imagination...