What is the source for eating dairy on Shavous, and what are the reasons for it?
Earlier authorities (Kol Bo: 52, Seder Hayom: Shavous) mention the custom of eating dairy on Shavous, but they say one should eat it along with honey based on the verse in Shir Hashirim (4:11). This custom is brought in the Chok Yaakov (494:9), Aruch HaShulchan (494:5), and Mishna Berura (494:13).
However, these days, most eat dairy even without honey.
What is the basis of that custom?
The Rema (494:3) says that just like on Pesach, we remember the Korban Pesach and Chagigah on our Seder plate, so too on Shavous, we eat meat and dairy to remember the Shtei Halechem, the two loaves of bread that were offered in the Beis Hamikdash on Shavous.
Mishnah Berura (494:14) explains that since eating the same bread for a meat and milk meal is forbidden, we are forced to bring two separate loaves to the meal; thereby, we remember the Shtei Halechem.
Aruch HaShulchan (494:5) brings the verse in Bamidbar (28:26) about Shavous:
"הַבִּכּוּרִ֗ים בְּהַקְרִ֨יבְכֶ֜ם מִנְחָ֤ה חֲדָשָׁה֙ לַֽיהֹוָ֔ה בְּשָׁבֻעֹ֖תֵיכֶ֑ם"
who's first letter of the last 3 words spell chalav- milk.
The Mishnah Berura (494:12) provides another reason: when the Jewish people accepted the Torah on Shavous, they now had new halachos regarding meat. They could not eat meat without making numerous preparations, like checking the knives and kashering the pots. Until they completed those tasks, they ate only dairy. To recall this, we eat dairy on Shavous.