Mitzvas simchas yom tov triggers a dispute between Rabi Yehoshua and Rabi Eliezer. According to Rabi Yehoshua the Torah mandates chetzyo la'Hashem v'chetzyo lachem, i.e. half the day must be devoted to davening and learning, and half to eating and drinking. According to Rabi Eliezer, on the other hand, the Torah does not insist on any such balance. One may, if he wishes, divide the day as prescribed by Rabi Yehoshua. But, if he so desires, one may also devote the entire day either la'Hashem (davening and learning) or lachem (eating and drinking).
Famously, the gemara adds a caveat: "ba'Atzeres kulai alma modu d'b'enan lachem. Mai ta'a'ma? Yom shenitna bo Torah l'Yisroel - on Shavuos even Rabi Eliezer insists that part of the day be devoted to eating and drinking. Why? Because the Jewish people received the Torah on Shavuos"
The question is obvious: eat, drink and be merry...for today we accept the Torah!?
The answer is to be found in the Torah's approach to the physical. "Barasi yester hara u'barasi lo Torah tavlin - Hakadosh Baruch Hu says to the Jewish people, 'I create the evil inclination. I created the Torah as seasoning / spices for it.'"
The yetzer hara - the inclination for the physical - is not something that needs to be suppressed. Tavlin do not suppress; they lend flavor. The analogue: our physical inclinations are to be disciplined and thereby redeemed. The discipline and consequent redemption of the physical is an integral part of Torah. To reflect this crucial part of Torah we do indeed eat, drink and (with the appropriate koved rosh) make merry on Shavuos.
But the significance of eating and drinking on Shavuos reaches yet deeper. Ramabam penned a remarkable responsum to an inquiry as to the permissibility of listening to Arab music and lyrics. In essence, his response: for various reasons, most decidedly assur. Rambam concludes on the following note: our mandate is to be a goy kadosh. This means to devote and direct all our energies to the source of Hakadosh Baruch Hu. In particular, Rambam fails to see how listening to Arabic music will advance that sacred agenda. In general, we should strive that everything we do should be calculated and geared to avodas Hashem. Rambam develops this idea in Perek 3 of Hilchos De'os as well as identifying it with the maxim of Chazal, "v'chol ma'asecha yi'hi'yu l'sheim sho'mayim" and the teaching of Shlomo Hamelech, "bechol d'rachecho da'ei'hu." These are all-encompassing mandates, even activities such as eating and drinking should be calibrated and geared towards avodas Hashem. Eat, drink and be healthy so after breakfast you may go to the beis medrash and concentrate upon your learning.
This pivotal idea is also reflected in the mandate of eating and drinking on Shavuos.