Why a soul needs to be in a state of grace when receiving the Eucharist?

What is the reasoning behind the Catholic Church teaching that only people in the state of grace should be allowed to receive the Holy Eucharist? Lately, I heard a statement: > The Eucharist: A prize for the just or medicine for sinners? > > The Eucharist is no gold star on the forehead of “good” Christians, > but an undeserved gift to strengthen pilgrims who stumble along > through life with their gaze fixed on heaven. What is this teaching based on that one needs to be in the state of grace and why?