From sermon by Fr Philip Neri Powell OP (here):
…fasting and mourning brings to mind all that is missing.
Bringing to mind all that is missing for Christians is about more than mere remembering. Memorials are common markers for noting people and events. We memorialize war heroes, literary giants, athletes. Photo albums, keepsakes, tombstones bring to mind both the living and the dead. For the disciples of Christ, fasting brings to mind both Christ’s presence and his absence; both his life and his death. When we fast, we recognize—thing about again—our dependence on God, our adoption as His children through Christ. And we remember—become members again—of his Body, the Church. When we fast from food, we fast for a memory and that memory is made present, made real. When we fast from whatever it is we are fasting from, we fast for a reason, a purpose and that reason is to mourn, to lament the absence of all that is missing. Christ is not with us, so we fast. Yet, he is always with us, so we feast. We cycle through the year fasting and feasting, remembering and recognizing that he is gone, he is here, and he will be among us again. At the bottom of the question about fasting lies humility—our absolute dependence on God. Without Him, there would be no feasting at all.