"It's most important.

You can't have this letter unless I KNOW.
It's the very first time Laddie ever trusted me with a letter, and I just can't give it to the wrong person." "Then why don't you leave it where he told you ?" "But you have gone and found the place.

You started to take it once; you would again, soon as I left." "Look me straight in the eyes, Little Sister," said the Princess softly.

"Am I like a person who would take anything that didn't belong to her ?" "No!" I said instantly.
"How do you think I happened to come to this place ?" "Maybe our woods are prettier than yours." "How do you think I knew where the letter was ?" I shook my head.
"If I show you some others exactly like the one you have there, then will you believe that is for me ?" "Yes," I answered.
I believed it anyway.

It just SEEMED so, the better you knew her.