In Musaf for Yom Tov, we say ותתן לנו ה' אלקנו באהבה [שבתות למנוחה ו] מועדים לשמחה את יום החג הזה זמן .... [באהבה] מקרא קדש וכי (additions for Shabbos in square brackets and italics.) Why do we need another באהבה for Shabbos when we already had one earlier in the same sentence?
The first "באהבה" refers to G-d's love, the second one refers to our love.
According to the Levush (brought here), we add the 2nd "באהבה" on Shabbos, since we willingly, lovingly accepted Shabbos at Marah, before Matan Torah. The holidays were given to us at Sinai, where the Talmud tells us we were coerced into accepting the Torah.
G-d's love , on the other hand, was always there, both when he gave us the commandments at Marah and at Matan Torah.
There is a letter from the Lubavitcher Rebbe that further elaborates on this, but I don't know where it is offhand.