Sometimes you think your house is being taken over by books and teacups. You can't cross the parlor without tripping over them. They are the true owners of these rooms, not you.
You go through your bookshelves. To separate the wheat from the chaff? No no, each spine you pull off the shelf puts a weight on your consciousness. You're not going to read all of them, but you want them around you like photos of your loved ones on the wall. You want them on your shelves like trophies of your good taste and verbal promiscuity.
Postcard Memoir you are not sad to see go, in fact you think of the book in shades of contempt. Woe is I you consider keeping as a grammar reference. But hey, isn't that what the internet is for?
You pull out Houellebecq's The Elementary Particles. You thumb through the pages, looking at the understated pencil lines around the words that resonated with you. Too many pages of resonance to quickly jot down your favorite passages. You put it back on the shelf.
You put Camus's The Fall in the To Sell pile with a sigh. You often think of it with nostalgia, and sometimes when you speak of it you pretentiously call it La chute, as if you and it were on a first-name basis, as if you could fool people into thinking you knew French. But it's unlikely you'll read it again, and so into the To Sell pile it goes.
No, you can't keep them all: You need the money that comes from selling them. You want less boxes and less weight when you move out; you want the cloying freedom that comes from lack of possessions. The tabula rasa of empty bookshelves. But really, you need the space for new loves.