Musar к Шир ха-ширим 3:11

There is a Midrash Chazit on Song of Songs 3, 21 in which Rabbi Shimon ben Yochai asks Rabbi Eliezer son of Rabbi Yossi whether he had heard any comment about the crown his mother had made for him. Rabbi Eliezer replied in the affirmative. When asked what that comment was, he replied that it was a parable describing a king who had only one daughter whom he loved excessively and whom he called בתי, "my daughter." In the course of time he took to calling her אחותי, "my sister." Still later he started calling her אמי, "my mother." This describes the developing relationship of love between G–d and Israel. It is reflected in Psalms 45,11 where G–d is described as addressing Israel as "listen daughter, and incline your ear." In Song of Songs 5,2 Israel is addressed as "open your heart to Me My sister;" in Isaiah 51,4 Israel is addressed as עמי ולאומי, and the latter word is spelled Immi, "my mother." Upon hearing this Rabbi Shimon ben Yochai stood up and kissed Rabbi Eliezer on his head and said: "if I had only come to hear this word from you it would already have been worthwhile."