There are a number of answers to this question. The first answer is that most poskim hold that there is no chiyuv simcha on Shabbos. It could very well be that the above-mentioned Sifri would even agree. It is proper to have simcha on Shabbos, but there is no chiyuv to have simcha. The Gra explains the reason we use the terminology of yismechu bemalchuscha is going on the future; in the zchus of keeping Shabbos properly, we will be zocheh to be happy with the Shechinah Hakedosha. The Rosh, at end of Perek Daled in Rosh Hashana, and the Mordechai say that the posuk “On the day of happiness” is not referring to Shabbos, but rather to Rosh Hashana. This is why we don’t say lesasson ulesimcha in the birchas Haftorah on Shabbos.
The Baal Halachos Gedolos argues with this point and says that there is a chiyuv to have simcha on Shabbos. In his count of Mitzvos Asseh he counts oneg Shabbos and simcha
The third tirutz is based on the Shailos U’tshuvos Lev Chaim who says the reason there was simcha on Shabbos was because they blew the trumpets during the Korbanos on Shabbos. The Simcha came as a result of the korbanos. Therefore, in tefilla which corresponds to the korbanos, we say simcha, but not that one has a chiyuv of simcha by eating meat or drinking wine,
The fourth and final answer could be that on Yom Tov we have a chiyuv of simcha that is physical, whereas Shabbos is a celebration to remind us that Hashem created the world in six days and rested on the seventh day. The ikar mitzvah of Shabbos is not to do melacha. We are commanded to leave the physical and strive to reach a spiritual level of simcha and kedusha, which is why we have a neshama yeseira on Shabbos
May we be zocheh to be happy on Shabbos the correct way!
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