As we wrote earlier, it is uncommon for Rashi to write an introduction to his comments. In our case, before commenting, he announces that it is beautifully expounded in a Midrash.
We have often stated that Rashi's primary purpose is to explain Peshat, the simple meaning of each verse in the Torah. We know this because he says as much on many occasions. There are also deep mysteries of the Torah, which are alluded to in Rashi's words. These are known as the "wine of Torah." However, Rashi is, first and foremost, a guide to the beginning student. Why would he present something from a Midrash as his first explanation? Not just that, but he makes a point of emphasizing that it is beautifully expounded there.
There is usually a reason that Rashi offers two explanations of the exact words. Generally, it is because there is a problem understanding the first explanation. The second explanation does not have that particular difficulty. However, he teaches it first because it is closer to Peshat. However, his first comments are not Peshat; they are a Midrash.
