When something moves you, you want to keep that feeling.

"I started keeping Commonplace books—journals with lists of copied-out quotes—as a teenager. I spent a lot of time writing in journals and also reading—getting to know other authors. When I was so moved by them, I wanted to do something about it, so I would write down the lines and passages I loved, as a way of remembering them, as well as sort of studying them. I think the act of writing out lines from books helped me absorb that language longer and more deeply. When something moves you, you want to keep that feeling. Writing a line down again is a way of holding onto it."

Why You Never Truly Finish a Book / Susan Minot