In preaching God must give us what we want to give to others.

"Oh Lord my God, hear my prayer, and let Thy mercy regard my longing, since it burns not for myself alone, but because it desires to benefit brotherly charity; and Thou seest into my heart that so it is.  I would sacrifice to Thee the service of my thought and tongue; and do Thou give what I may offer to Thee."  - Augustine, Confessions.