At the beginning of Parshas Yisro the Torah describes how after Yisro decided to convert to Judaism, he brought various korbanos. "Vayikach Yisro olah u'zevachim l'Elokim - Yisro brought both a korban olah and korbanos shelamim" (Shemos 18:12). Why did he bring both types of korbanos?
Perhaps the answer is that this double korban symbolized Yisro's transition from non-Jew to full-fledged ben Yisrael. The halacha is that a non-Jew cannot bring a korban shelamim, only a korban olah (Menachos 73b). Some explain that this is because according to the non-Jewish world's perspective, kedusha requires a total separation from physicality; to live a life of holiness, a person must deny himself physical pleasure. For a non-Jew, the korban olah is the only way to serve Hashem because a non-Jew feels that a spiritual life requires total sacrifice.
However, the Torah has a different perspective. While there certainly is room for a korban olah which is completely burnt on the mizbeach, there is also a place for a korban shelamim, in which part of the korban is burnt on the mizbeach, part is given to the kohein, and part is also eaten by the owner. The korban shelamim shows that the Torah believes that man can partake of the physical world, he can enjoy physical pleasures like eating and drinking, and still be serving Hashem. Kedusha does not require a person to abstain from the physical world. It requires that he elevate and sanctify the physical world. By bringing both an olah and a shelamim, Yisro demonstrated that he understood this message.
This idea can also help explain a puzzling Gemara (Pesachim 68b.) which says, "All agree that to fulfill the mitzvah of simchas yom tov on Shavuos, one must have some physical pleasure because on Shavuos the Torah was given to the Jewish people." On all other yomim tovim, the Rabbis argue as to whether a person can choose between total immersion in spiritual pursuits (kulo l'Hashem) and complete involvement in physical activities (kulo lachem), or rather he should split the day chatzi l'Hashem v'chatzi lachem - he should engage both in spiritual endeavors like davening and learning Torah, as well as physical activities like eating and drinking. But on Shavuos, everyone agrees that some physical enjoyment is necessary.
At first glance, the opposite seems more logical. After all, Shavuos is the day that the Jewish people received the Torah, a day on which we celebrate the value of ruchniyus in our lives. Why must there be some portion of lachem on that day? If anything, everyone should agree that on Shavuos one can choose the option of kulo l'Hashem to fulfill the mitzvah of simchas yom tov!
The answer is that precisely because Shavuos is the day of kabbolas haTorah we have to eat and drink to celebrate the yom tov because Shavuos is a day that we declare our commitment not only to learning Torah, but to living a Torah lifestyle as well. And there is no better way to demonstrate the Torah's perspective on life than by elevating ourselves through eating and drinking (see Beis Halevi).
As Jews, the ultimate level we can aspire to is not to separate ourselves from the world, but to engage in physical activities - even the most mundane - and imbue them with a sense of kedusha. When we eat and drink l'shem shomayim, in a refined way, when we dedicate some of our resources to tzedaka and hiddur mitzvah, we demonstrate that we have internalized the message of the korban shelamim. We do not have to abstain from physical pleasures in order to reach the ultimate level in avodas Hashem. All we have to do is live for a higher purpose.