Source HAPPINESS vs SADNESS
[19a middle] Mishna: Mourning is cut short by YomTov (Pesach and Sukkos). Shabbos however, does not interrupt the Shiva/Shloshim. Meiri: Shabbos is all about Oneg (physical pleasures like eating) and this is possible to have even during mourning. YomTov’s aspect of Simcha cannot coincide with Aveilus. R’ Soloveitchik: Simcha is not just about outward activities like Oneg (eating meals, drinking wine, say Hallel), and Aveilus is not just about outward activities (like sitting on a low chair, being uncomfortable). Both Simcha and Aveilus are actually fulfilled by using our hearts and emotions. The Torah/Halacha never demands contradictory feelings from people, and that is why the Joy of YomTov knocks out the Mourning of Aveilus [Rambam even says there are no aspects of mourning allowed on YomTov at all].
view E’s Lesson: We should recognize that whenever Hashem or the Rabbis have set forth laws, obligations or challenges, we must be able to accomplish the required tasks. We should never doubt our abilities and remember that we are only here to accomplish what we are capable of doing.