The Rama rewards the Minhag to eat dairy on Shavuot. Five reasons that are commonly given for this Minhag can be found in טעמי מנהגים. The first explanation discussed is that eating dairy reminds us of the שתי הלחם that were brought on Shavuot. Hence, we wash, eat and recite Birchat Hamazon over a dairy meal, and subsequently we wash and eat a flieshig meal and recite Birkat Hamazon. The two meals represent the שתי הלחם. Another reason is that we should eat honey and milk, as Shir Hashirim describes, דבש וחלב תחת לשונך “we had milk and honey under our tongues” when we received the Torah. A third reason is that until מתן תורה Jews were permitted to eat meat that was not slaughtered according to Jewish law and animals that were unfit by Jewish law. However, when they received the laws of Shechita and forbidden foods, they did not have the opportunity to slaughter and prepare Kosher meat so they had to eat dairy. Thus, we eat dairy to remind us of that first Shavuot. Another reason mentioned is that one of the names given to Har Sinai was מנהון גבנומים, clean as cheese. Because מנהון גבנומים was where we received the Torah we eat cheese on Shavuot. A fifth reason mentioned is the fact that the Torah tells us to be humble and eating dairy is a modest meal, whereas eating meat is not as modest.