We fast on Yom Kippur because the Torah commands us to "afflict our souls" on this Day of Atonement, which is observed as a day of intense self-reflection, repentance, and spiritual cleansing. Fasting by abstaining from food and drink shifts focus away from physical needs, allowing the soul to connect deeply with God and engage in sincere repentance and inner awakening. It symbolizes purification, sacrifice, and atonement, aligning the individual with a higher spiritual state akin to angels who do not eat or drink. The fast serves as a physical discipline that enhances the ethical and spiritual goals of Yom Kippur, helping people detach from bodily demands and focus on their relationship with the Divine and their moral conduct.