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
When the meadows laugh with lively green
And the grasshopper laughs in the merry scene
When Mary and Susan and Emily
With their sweet round mouths sing "Ha, ha he!"
When the painted birds laugh in the shade
Grasshoppers  have multiple symbolic associations, including longevity, happiness, peacefulness, and good health.   In this context it is most likely that Blake chose it because of its characteristic leaping movement, suggesting joy and energy and good health. 

Laughing Song