Starting with his epigraph, Thoreau announces that he wants to wake us up. That's why it's not really surprising that the tone in the book is generally exuberant, as if Thoreau was shouting in your ear through a megaphone. He knows what he's doing and he's not going to apologize for it. As he writes:
If I boast more than is becoming, my excuse is that I brag for humanity rather than for myself; and my shortcomings and inconsistencies do not affect the truth of my statement. Notwithstanding much cant and hypocrisy – chaff which I find it difficult to separate from my wheat, but for which I am as sorry as any man – I will breathe freely and stretch myself in this respect, it is such a relief to both the moral and physical system; and I am resolved that I will not through humility become the devil's attorney. (Economy.71)
So yes, Thoreau sometimes goes off into excess and hyperbole, and he suggests some occasionally silly ideas like measuring a man as if he were a pond, but it's all in the name of just being who he is in all his originality. He's sure of himself (bombastic), yes, but enjoying every moment (and helping us enjoy it, too).