I am no expert, just someone trying to get better. I can tell you that my children always comment on the stories in these letters. They never mention the lesson itself. I think stories need to pull you in, connect in some way. This might be an emotional trigger or it might simply be entertaining. Allowing the reader to imagine the scene in their head helps. We think in images, not words. For example, simply stating facts will lose nearly everyone. But craft a narrative and there is a chance to pull someone in.