Do you share all of your recipes?

When you are given a compliment on a food item at a pot luck and someone wants the recipe do you share or do you play dumb ? At a recent outing I tasted a very good chocolate cake (very moist) that had a delicious chocolate frosting(not too sweet or too bitter) the guy that made it told me he would email me the recipe, he did but the finished product was not anything like what he made. Why is it so hard for some people to share a recipe?