The Shach on the Torah, in the name of the Shaarei Orah, explains that Yitzchok loved Esav because he saw that Yaakov’s children would one day sin and deserve to be punished in Gehinom. In order to spare them this punishment, Yitzchok chose for them the lesser punishment of exile, and chose Esav to carry out the exile. In this manner, he followed in the footsteps of his father, Avraham Avinu, who also chose exile for his children (Bereishis Rabbah 44:21). This is the meaning of the Midrash in Shir Hashirim. “Do not arouse the love before it is desired” is understood by the Midrash to mean: “Do not use actions to upset the love of Yitzchok for Esav before the desire of the patriarch [Yitzchok] has been achieved.” In other words, Yitzchok’s love for Esav is behind our exile, and it is forbidden to use actions to upset that love.