The Fisherman's Theory of Book-Buying: You will never regret the book you bought, but you will always regret the one that got away.
The National Debt Theory of Book-Buying: You will never have read all the books you own, but any given book will be read eventually.
The Chemist's Theory of Book-Buying: Books obey the laws of gases: they expand to fill all available space.
The Gardener's Theory of Book-Buying: No matter how much you weed a book collection, it will always grow back.
The Pharaonic Theory of Book-Buying: Build a pyramid and read them all in the afterlife.
This was from the December 2006 issue of NYRSF by Darrell Schweitzer
I've been culling and culling the books for five years now and while it's getting more manageable and more narrowly focused, there's still a lot of cruft floating around :)