When the green woods laugh with the voice of joy
And the dimpling stream runs laughing by
When the air does laugh with our merry wit
And the green hill laughs with the noise of it
William Blake opens the poem with words that echo teh Old Testament book  Jeremiah 33:11 : 
 “ the voice of joy and the voice of gladness, the voice of the bridegroom and the voice of the bride, the voice of those who say, "Give thanks to the LORD of hosts, For the LORD is good, For His lovingkindness is everlasting ” 
 This establishes the theme of the poem, in which Blake or the speaker and the  anthropomorphised  environment revel in joy that closeness to nature can bring.
Laughing Song