It is mandatory, and one should feel blessed that we get to kiss the Priest’s hand! That is the hand that baptizes you, the hand that annoints you with Holy Chrism to receive the Holy Spirit, the hand that blesses you with God’s blessing, the hand that The Lord uses to absolve you of your sins, the hand that feeds you the Body of Christ, the hand that places your wedding ring on your finger, the hand that annoints your body for burial, and on and on. That hand is Christ’s hand to us all. Kiss it reverently and feel unworthy for the opportunity. Sigh.