The mourning and fasting of Tisha B’Ab (Ninth of Ab) are seen as a way to cleanse and afflict the soul (Neshamah) and empty oneself of the pain and sorrow of the past, making room for Divine Lights (Sefirot) to re-enter it.
Thus, Tisha B’Ab is a spiritual mechanism that allows us to shed the negativity of the Sitra Achra and refocus on the efforts of our Neshamah’s mission of descending through the worlds of negativity and then returning to the Primordial Tehiru.