(hat tip Sh)
The letter and beautiful explanation is Here
I don’t have time to translate it but the Rebbe wishes Rav Soloveitchik a good Yom Tov using the language of his father in law the Rayatz which included accepting the Torah happily. When he came to sign the letter he explained the word happily ie בשמחה
The difficulty is we are meant to be in fear. What does the emotion of happiness have here. Based on a Gemora in Brachos, Rishonim and the language of the Shulchan Aruch HoRav, it is explained that fear most certainly has its place during learning Torah, but at three other stages the emotion of happiness is appropriate. One of these is on Shavuous when we accept the Torah.