I was also ready the brook ,
the message i got is that ,
the poet says how the life of human begins and how it travels throughout his life how he comes to his childhood again after becoming old
how his speed means the energy is so high when he is falling from the mountain means how the energy is high when he was a child how it flows in a zig-zag manner which shows the mood of the child or the young man and after all after coming to the plains the brooks is at the least level means that the old-age comes and he is not energetic now. this poem is beautifully personified .the brook or the small river in the poem represents the life of a human being.
the message conveyed is nothing but how th peoples life changes from one stage to another stage is described in the poem.