Chapter nine is a sequence of four practical observations followed by a conclusion, and it is among the most immediately usable passages in the book.
Filling a vessel to the brim is worse than stopping in time, because a full vessel spills. Grind a blade too keen and the edge will not last, because a fine edge is fragile. A hall full of gold and jade cannot be guarded, because wealth attracts what takes it. Wealth and rank held with arrogance bring their own ruin, because arrogance produces enemies.
None of these is mysterious. Each is a case of a good thing pursued past the point where it remains good, and in each the failure is produced by the excess itself rather than by bad luck. The chapter is making a claim about limits: that most goods have an optimum well short of the maximum, and that the damage from overshooting is not incidental but structural.
The idea recurs throughout the book. Chapter forty-four asks whether the loss from hoarding is worth the gain. Chapter forty-six names not knowing what is enough as the greatest calamity. Chapter thirty says that things which grow strong then grow old, and calls that contrary to the way.
What makes chapter nine distinctive is that it argues entirely from ordinary experience, without invoking the way at all until its final line.