[...] no man is the lord of any thing,
Though in and of him there be much consisting,
Till he communicate his parts to others:
Nor doth he of himself know them for aught
Till he behold them form'd in the applause
Where they're extended; (3.3.96-101)
Ulysses wants Achilles to feel worthless so the guy will get back out on the battlefield. That's why he tells him he's reading a book that says a man is only as good as his reputation. (Which, seriously? Achilles falls for this?) If a man doesn't have a lot of friends and admirers to give him props, then he's totally worthless. Clever Ulysses, because this confirms what Achilles already believes.