Chazal recognized that it would not be healthy for us to be in constant state of mourning throughout the year and therefore designated a small period of time during which to express mourning (and many hold that they also confined all future communal mourning to be observed during this period). They established a specific day (or weeks, as it were) only to limit our mourning to a healthy amount of time.
Fine. Substitute Torah Learning and Shavous — Yom Kippur and repentence — Pesach and remembering the Exodus.
Same questions apply.
The Wolf