What is, however, “flattery” of the evil sort is when a person displays insincere affection towards his fellow, while in truth his mouth is not saying what he is really thinking. With his mouth one speaks peace with his fellow, but inside of him he lays his ambush (Yirmiyahu 9:7). But even if the person is not actually seeking the other fellow’s evil but is only showing more friendship and honor than he truly feels in his heart — he too has failed in the sin of flattery. Still, our Sages allowed a husband to flatter his wife in this manner for the sake of peace in the home. They likewise allowed flattering one’s rav (Torah teacher) so that the rav will teach him Torah.
כ״א באייר ה׳תשפ״ב
My brother and my nation, take this Torah book, in this and like this learn well, listen and revival your soul, you will achieve joy and joy, in this world and next world. (from introduction of the author) 1