I have two friends who each have a recipe they won’t share. One is for a sherry cake that this friend always gives as Christmas gifts, and the other is a punch recipe that is a family recipe. I don’t know if the sherry cake recipe is a family recipe or not. It kinda bugged me initially (I told the one friend that I would never make her sherry cake for anyone but myself, but she would not relent). Sometimes I think people are reluctant to share because the recipes are super easy, and they are kind of embarrassed at that? Maybe? I’m pretty sure her recipe has a butter golden cake mix in it. I’ve had other sherry cakes that also did but none are as good as hers. So, there is definitely “something” to hers. I think for both of these recipe, they each consider them to be their, “signature,” thing.
But, even though their not sharing bugged me, I got over it. I think the risk of the, “It’s not written down,” reply is that someone will say, “Oh - well, will you please write it down the next time you make it?”
There are certain cooking skills which truly cannot be written down (these are on the opposite end of the spectrum of the super simple ones). They require a skill, a technique, a “feel,” that can’t be captured in the written word. One family friend makes divinity. It is her thing. She happily shares her recipe, but her daughter has observed her making it and helped her make it a gazillion times and still can’t replicate it. So, if the “unwritten” recipe involves something like that, and the other person asks you to please write it down, then you run the risk of sounding like a superior cook. “Well, I can write it down, but you won’t be able to make it as well as I do.”
I always share recipes, but I certainly understand if people don’t want to. I might not always like it, but I get it.