When Esau came back from hunting, exhausted, and asked Yaacov for some lentil stew, Yaacov asked him to sell him his birthright in exchange. R’ Munk wrote that the continuation of the spiritual heritage of Abraham was the objective, understood by Yaacov. The responsibility would be to ensure the permanence of family traditions, something which Esau found unacceptable. It was more of a duty than a right, and would curb Esau’s freedom to follow his interests. Ramban interpreted Esau’s lack of interest in the birthright as due to it not having material or judicial advantages, but only the privilege of worship.
R’ Shmuel pointed to the similarity of the words Bechora (birthright) and Beracha (blessing) so when Esau said “what good is the birthright to me?”, all the more so to ask what good would the blessing have been to him.
When Yitzchak had aged and was ready to bless Esau, Rivka overheard his request that Esau prepare his favourite food, and she encouraged Yaacov in his tricking of his father into giving him the blessing of the firstborn. R’ Munk wrote that Yitzchak had become blind to Esau’s faults. Ramban wrote that the gift of prophecy had left Yitzchak, and he hoped he’d find the inspiration from the food. Yitzchak had believed that Esau would somehow improve and take up the mission of the firstborn son. R’ Munk wrote that Rivka was not convinced and that often the mother understands the nature of her children better than the father. She had also, when pregnant with the twins, had the revelation that the older would serve the younger. To allay Yaacov’s concerns about cooking 2 kids from the flock and clothing Yaacov in Esau’s garments, Rivka said “your curse be on me”. She was absolutely sure of herself.
R’ Munk wrote that the blessing was not given unconditionally but that by using the name Elokim, (G-d as Judge),in the first sentence, quoting Rashi that “if it is due to you, G-d will give it to you, and if not, He will not give it to you” . Rashi interpreted Yitzchak’s subsequent blessing to Esau as him being able to throw off the servitude to Yaacov’s descendants when “Jewry transgresses against the Torah”. Because of Esau’s anger, Rivka advised Yaacov to go to her brother Laban in Charan. Yitzchak then formally blessed him with “the blessing of Avraham”, making him the heir to the Divine blessings granted to his grandfather. He also told him not to take a wife from amongst the Canaanite women.This was the point at which Esau understood that he should chose a wife who would meet his parents’ approval and married Mahalath, a daughter of Ishmael.