The subject of this poem is Wordsworth's daughter, Catherine, who died very young.
Surprised by joy - impatient as the wind
Surprised by joy – impatient as the wind
I turned to share the transport – Oh! With whom
But thee, long buried in the silent tomb,
That spot which no vicissitude can find?
Love, faithful love, recalled thee to my mind –
But how could I forget thee? - Through what power,
Even for the least division of an hour,
Have I been so beguiled as to be blind
To my most grievous loss? – That thought's return
Was the worse pang that sorrow ever bore,
Save one, one only, when I stood forlorn,
Knowing my heart's best treasure was no more;
That neither present time nor years unborn
Could to my sight that heavenly face restore.
For a nice explanation of the poem, see this essay in The Guardian. Here is a bit of that piece.
This sonnet activates a series of "pangs": the moment of delight, the desolate realisation that the one person with whom the speaker wants to share it is irrecoverable, the pang of guilt that the beloved child could have been forgotten, and, finally, the remembered moment of desolation. The natural event, the source of the joy, un-described but beautifully left to our imaginations, is quickly erased. Nature, for once, fails the poet. The sonnet seems to travel a considerable distance, though it turns back on itself to look again at the moment of anguish, "when I stood forlorn,/ Knowing my heart's best treasure was no more." It blossoms early and vividly but it appears to be prematurely concluded, or, at least, rapidly transformed from immediacy into memory.