Why is having a good time the first thing they did with Chet HaEigel? Perhaps because this was their concept of what religion is all about. Like we see that: ת ח תַּאֲשֶרַּלֹאַּעָּב דְתַָּּאֶתַּה'ַַּּ אֱלֹקֶיךַָּבְשִמְחָּהַּוּבְטוּבַּלֵּבָּבַַּּתבאַּכחַּמז not serving Hashem with Simcha is the only sin mentioned in the whole Tochacha (the horrible punishments mentioned at length in Parshas Ki Savo), i.e., what is Yiddishkeit all about? Simcha! And what did Moshe tell Paroh when he wanted to take Klal Yisroel out of Mitzrayim? וְיָּחֹגוַּּ לִיַַּּ ב מִדְ בָּרַַּּשמותַּהַּא “In order to make a Chag for Hashem in the Midbar”.
We want to leave Tum’as Mitzrayim so we can have a beautiful good time and a Yom Tov in the Midbar! Unfortunately, there are rules, on how and when to enjoy yourself, and in a kosher way; and with Chet HaEigel they had misplaced enjoyment. But either way, we see how having a good time is the Yesod of the: בְרִ יאָּה creation; like the Mesilas Yesharim says: הָּאָּדָּםַּלֹאַּנִבְרָּאַּאֶלָּּאַַּּ לְהִתְע נֵּגַּע לַַּּה' a person was created only to have pleasure in Hashem.
