With Love And Will

Today is Maran HaRav Hutner's yahrtzeit. In shul I picked up a copy of his sefer of letters to learn something for a minute or three after davening. This is what I saw...

On Shabbos we say in davening באהבה וברצון - with love and will. On Yom Tov we omit these words. Why?

The sefarim explain that at Har Sinai we were coerced to accept the Torah - כפה עליהם הר כגגית. Chazal say that in a place called Marah the Jews were given Shabbos   So Yom Tov was accepted against our will at Sinai while Shabbos was accepted willingly at Marah.

Maran was not convinced by this explanation.

A] The Rambam famously says that we don't keep mitzvos because of any event that preceded Har Sinai. For example, we don't refrain from eating the גיד הנשה because of what happened to Yaakov Avinu with the angel but because we were commanded at Sinai. So one cannot say that we keep Shabbos because we were commanded at Marah. 

B] In davening we say אז סיני נצטוו עליה  - at Sinai we were commanded. This is seemingly precluding the approach that we keep Shabbos because we were commanded at Marah.  

C] Another question - but I don't remember:-)!

See here for more on this topic.