I'm a children's librarian, and some of my favorite words to hear are:

"This is just the right book."

"I got an "A" on that report!"

"More stories, PLEEEEEEEEEASE!" (Said by a roomful of pre-kindergartners with their best manners on.)

"I remember you! You're the one who told that story about..."

"Hey! Library lady!" (the best occasion was on the fourth of July watching fireworks five miles away from my branch.)

But I'll admit, my favorites are usually "The library will be closing in five minutes, please bring your books up to the front desk to be checked out."