The scyboy in the poem is not a happy child. What makes him unhappy? Why does he compare himself to a bird that lives in a cage or a plant that withare when it should blossom. 

I love to rise in a summer morn,  when the birds sing on every tree , the distant huntsman winds his horn and the sky lark sings with me. O! What sweet company. But to go to school in a summer morn, o! It drives all joy away; under a cruel eye out worn, the little ones spend the day, In signing and dismay How can the bird that is born for joy, sit in a cage and sing. How can a chil when fears annoy, but droop his tender wing and forget his youthful spring. O! Father and mother if buds are nip d and blossoms blown away, And if the tender plants are strip d Of their joy in the springing day by sorrow and cares dismay how shall the summer arise in joy or the summer fruits appear from the school boy by William Blake .