R. Hershel Schachter on TorahWeb:
Before G-d was prepared to give His Torah to the Jewish people He first wanted to know whether they were prepared to accept it. With great enthusiasm the Jews expressed their desire to both accept and observe all of the laws of the Torah. Then according to Talmudic tradition (Shabbos 88a), G-d pressured the Jewish people to accept the Torah, and forced it upon them against their wishes.
The commentaries on the Talmud all wonder, why it was necessary to force the Torah upon the Jews if they had already enthusiastically expressed their willingness to accept it? The Medrash Tanchuma (to Parshas Noach) elaborates upon this aggada and distinguishes between the different parts of the Torah. The people were prepared to accept both G-d's written Torah, and all the halachos l'Moshe miSinai – transmitted directly from G-d. Their response to Moshe was that “kol asher deiber Hashem na'aseh” - that all that G-d had said we are prepared to accept. But the bulk of the Oral Torah is really what the Talmud and the Rambam refer to as “divrei Sofrim”, halachos which were developed over the centuries with much rabbinic input. The rabbis were licensed to employ the various “middos shehaTorah nidrehses bohem” to read (so to speak) “in between the lines” of the Torah in order to present a fuller picture of each of the mitzvos. This the Jews at Har Sinai were not prepared to accept. They felt that this was not Divine! This is a human Torah, and all humans can err. Why should they agree to be subservient to the idea of other human beings? Who says that another is so much more intelligent than I? Each Jew should be entitled to interpret the law according to his own understanding!
And it was this part of the Torah that G-d had to force upon us. Whether we like it or not, G-d expects us to follow the positions set forth by the rabbis in interpreting the Torah. Not until years later, after the story of Purim occurred, did the Jewish people as a whole fully accept this aspect of rabbinic authority. It was at that time that Ezra and the Anshei Kneses Hag'dolah set up the entire system of the Torah sheb'al peh as we know it today. They formulated the text for all blessings and prayers, kiddush and havdalah, the system of thirty-nine categories of melacha, etc., along with many rabbinic enactments. Klal Yisroel at the period of the beginning of the second temple wholeheartedly accepted all of these formulations and innovations of their rabbonim.
 See essays by Rav Moshe Zvi Neriah in Meorot Neriah, Purim, pp. 164-171.