(from the afore-quoted section of Lewis' The Four Loves)
It seems to me that this section is getting into the sense of the water brotherly affection in Stranger in a Strange Land - the ideal that a water brother can ask any other water brother for anything, and the request will be honored, because ( in part), the request will never be unreasonable, when the nature of the relationship is truly understood and the level of Affection is at its purest.
In actuallity, to master the art of love to the point where you will never say something that inadvertently wounds is like a Platonic Ideal - unreachable. If we were telepathic... maybe. And I find in my relationship with Bill that I have to watch out for the mental path of "If he loved me enough he would know me well enough not to have said that in that way at this time." -- our knowledge of one another will get better over time, and we are both aiming for this kind of knowledge, but (especially since we as people change over time) our knowledge of one another will never be complete. And in many ways, it continues to be the things about Bill that surprise me that delight me the most, and things that he does delight me that even I (who presumably know myself well) could not have suggested he do.