Q. Why don’t we rejoice and make hakofos on Shavuos as we do in Simchas Torah, after all that is the day of the giving of the Torah and the wedding itself?

A. Our Sages (Seder Chag Hashevuos, Tel Talpios p. 18 et. al.) stress the great importance of being besimcha and in joy on the Yom Tov of Shavuos, since that happiness proves our commitment and devotion to the acceptance of the Torah. They say this explains why Rav Elozor (Pesachim 68b) teaches that all Sages agree that in Atzeres we require “Lochem,” meaning; “to rejoice in eating and drinking” (Rashi, ibid.) since it is the day that the Torah was given to us. Remah (O.H. 494: 2) explains that the reason for decorating our shuls and homes with greenery on Shavuos is to remember the “Simcha of Matan Torah.”
However, Meforshim also explain, why on Pesach there is no mention of the mitzva of Vessamachta Bechagecha, while on Shavuos the mitzva of rejoicing is mentioned only once and in Sukos twice, namely that on Pesach, people had not yet collected their grain and may still be worried and are not yet enjoying themselves properly. During Shavuos, when the grains have been harvested, they are already happy. More so on Sukos, when even the fruits have been gathered too, completing the years harvest. (Rabboseinu Ba’alei Tosafos, Yaino Shel Torah – Sukos p. 20). Toldos Yirzchok (ibid.) adds that it depends on the days of judgement that have already elapsed during this year. All four of them are over by Simchas Torah.
Horav Shlomo Miller Shlit”a explained, that having a wedding does not provide a guarantee that the marriage will be successful, only time will tell. So the real joy comes later on when we actually finish the reading of the Torah and confirm that the marriage is blooming. (See also Hadeah Vehadibur 20.)

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a