Why is the narrator in billy Collins's marginalia so tickled?

Tickled? The closest to being tickled the narrator gets is perhaps when he finds the margin note "Don't be a ninny" written in the margin of The Life of Emily Dickinson, and he pauses to try to imagine the person who would write that.

The one that strikes the narrator the most, although I wouldn't characterize it as "tickled" at all, is when he finds the note "Pardon the egg salad stains, but I'm in love" in a copy of Catcher in the Rye. He says that reading that made his loneliness deeper and amplified the world... "tickled" seems pretty shallow next to that.