There is a debate between Talmudic scholars, whether the holidays are for spiritual pleasures alone or whether the joy should be experienced physically as well, through eating festive meals and other pleasures. Regarding the holiday of Shavuot, the opinion is unanimous--we should enjoy the holiday through pleasurable food.
Even if someone had a bad dream and wants to fast to expiate the negative effects---which is permitted on Shabbat and holidays--is forbidden to do so on Shavuot. There is a special obligation to rejoice by eating fine food.
What is special about Shavuot? It is the day the Torah was given. Although the Torah is spiritual at its source, its purpose is to infuse our everyday physical lives with holiness. It is meant to be brought down to this world, not to remain as a theoretical concept. This is symbolized by eating. When we consume food, it becomes part of our flesh and blood. Similarly, when we study Torah it becomes part and parcel of our being. We express our pleasure at receiving the Torah through physical food and drink, because Torah is all about the fusion of the spiritual and physical.
The ultimate synthesis of the physical and spiritual will take place in the time of Moshiach. Then our physical appetites will not be an obstacle in the way of keeping Torah and mitzvot. Our bodily desires will be completely in sync with the needs of our soul. The physical will be purified and will be a perfect vehicle for the spiritual. For this reason, we celebrate Shavuot with physical delicacies, as a foretaste of the world to come.