The Torah tells us that when Ya’akov and Eisav grew up, Eisav became an “ish yodeya tzayid” – a man who knew trapping, while Ya'akov became an “ish tam, yoshev ohalim” – a complete man who abided in tents. “And Yitzchak loved Eisav, because trapping was in his mouth, and Rivkah loved Ya'akov” (Bereishis 25:27-28). The difference between Yitzchak and Rivkah in their respective love for their children appears, at first blush, to be disturbing, and needs to be understood.
Rabbi Avraham ben HaRambam explains that Yitzchak loved Eisav because he provided him with food, as the Targum explains. R. Avraham adds that some midrashim explain “ki tzayid befiv” (“because trapping was in his mouth”) to mean that Eisav trapped Yitzchak with his mouth by saying things that deceived Yitzchak and led him to believe he was careful about keeping mitzvos. This midrash clarifies the flow of the passuk: Yitzchak was able to retain his natural love for Eisav (despite his general deviation from God's path) because Eisav had been representing himself as being observant of the mitzvos. Even according to the midrash, however, Yitzchak's love came as a natural result of the physical benefit that he derived from him. Rivkah, however, loved Ya'akov beyond the natural love of a parent, because he spent more time at home, being a dweller of tents, and she therefore simply saw him more than she saw Eisav.
Rashi first cites the explanation of the Targum, just as R. Avraham does. He then cites the midrash, but he seems to understand it differently. Whereas R. Avraham finds a way to reconcile the midrash with the simple meaning of the verse, Rashi seems to understand it as being in contradiction to its simple meaning. Eisav, says the midrash, asked his father how one tithes straw and salt. In point of fact, only things which grow from the ground need to be tithed, and, so, Yitzchak was impressed by Eisav's scrupulousness in trying to fulfill the mitzvos. Rashi's apparent understanding of the midrash, explaining it to mean that Eisav deliberately fooled Yitzchak, is very difficult because it is in conflict with his approach to other midrashim about Eisav, as reflected in his commentary later in the parsha.
When Eisav discovered that Ya'akov deprived him of their father's blessings, he said in his heart: "The days of mourning for my father will draw near, then I will kill my brother Ya'akov" (Bereishis 27:41). Rashi there writes that this is to be understood 'as it sounds,' meaning, in its literal sense, that Eisav did not want to cause his father pain. Therefore, he would wait to kill Ya’akov until after his father's death. Rashi then points out that there are aggadic midrashim which explain the verse in several ways.
Nechama Leibovitz, in a seminal essay on Rashi's approach to citing midrashim, points out that there are often many midrashim to any particular verse, but Rashi very seldom tells us this. When he does, he means to reject those midrashim as not being in conformity to the simple meaning of the verse. In this particular instance, the other midrashim view Eisav as representing an additional stage in the development of evil in the world. Why did Eisav wish to wait until after his father’s death to kill Ya’akov? Eisav thought that when Kayin killed his brother Hevel, he made a mistake in not waiting until their father had passed away and could not further divide his estate. Therefore, Eisav decided to wait until after Yitzchak died, and then kill Ya’akov, so he would not lose his inheritance.
Rashi disagrees in that this view of Eisav represents him solely in a negative light, as a symbol of evil. Rashi maintains that Eisav, in fact, had a variegated personality, as he really did care for and honor his father. Therefore, Rashi felt that the midrash, while important for the message it conveyed, did not reflect the simple meaning of the Torah, which presents people as human beings, with all of their complexities.
In light of Nechama Leibovitz's insight, it seems very difficult to understand why Rashi in the beginning of the parsha would cite a midrash that seems to contradict the simple meaning of the verse, and, moreover, calls into question the love Eisav had for his father.
Rabbi Yerucham Levovitz answers that, in reality, Eisav was sincere in his questions. Indeed, both Rabbi Yosef Rosen (the Rogatchover Gaon) and Rabbi Chaim Kanyevski point out that Eisav’s questions were valid: there are situations in which one must, in fact, tithe straw or salt. One could add that Eisav specifically asked his father detailed questions about tithing because this was a mitzvah that Yitzchak took special care to keep, as pointed out by the Rambam in his Laws of Kings. Eisav, then, was not consciously trying to fool his father. However, one cannot ignore the fact that his scrupulousness in performing the mitzvos of honoring his father and tithing his crop were exceptions in his general demeanor.
Rabbi Levovitz says that this is the meaning of the words “ki tzayid befiv” – Eisav's mouth and his heart were not consistent. When speaking to his father and tending to his needs, he did and said all the right things. However, in his heart, he did not have an overall dedication to God. Ya'akov, on the other hand, is described as an “ish tam,” a complete man, in that everything he did was fitting and consistent with his overall demeanor. Ya'akov, unlike his brother Eisav, did not adopt stringencies in one area of divine service and completely neglect other areas; rather, he was a complete and integrated person, and thereby merited to be the one to carry on the tradition to future generations.