It has to do with what the Mass is: The sacramental re-presentation in time and space of Christ’s sacrifice on the cross. You might ask her, “If you could spend a half-hour to an hour every day for the rest of your life praying at the foot of Calvary, would you?” For a serious Christian, such a question should be as rhetorical as “Is the pope Catholic?” And that is why many Catholics go to daily Mass even though it is not required.