An old note can return a kindness its writer forgot performing.

Kindness is asymmetrical in memory. The person who wrote three sentences on a scrap of paper thought about it for a minute and never again; the person who kept it has read it forty times, sometimes on days when it was the only encouraging thing available. Nobody is keeping score, which is exactly why the discrepancy is so large. Small generous acts get filed as ordinary by the giver and as significant by the receiver.

Given that, the arithmetic favours writing things down. Send the note you were going to say out loud, and keep the ones you have been given somewhere you can find them.