Also, it’s an old custom to eat dairy foods on Shavuot.
Amongst the reasons for this tradition are:
1) On this day, when G-d gave us the Torah, we were elevated to a higher level of holiness. For this new level of holiness, the animals ritually slaughtered earlier were no longer fit for us. And the Torah was given to us on Shabat when it is forbidden to slaughter new animals ritually. Hence, not being able to eat meat, we ate dairy.
2) Every letter in the Hebrew alphabet also has a numerical value. The Hebrew word for dairy is “chalav.” Its numerical value is 40. This corresponds to the 40 days and 40 nights Moses spent on Mount Sinai studying the Torah with G-d, following G-d giving us the 10 Commandments.