A book borrowed for comfort may return with a different kind of gratitude than one borrowed for information.

What a book was needed for changes what its return means. Lend one for facts and it comes back with thanks the size of the favour; lend one because somebody is going through something and it comes back having been in a hospital chair or read at four in the morning. The lender may never learn which it was. Some gratitude is heavier than the occasion appears to warrant.

If something is returned with unusual care, let the warmth be received without pressing for the story. And when you lend at a hard moment, say there is no hurry. That sentence is often the actual gift.