When people say that are they thinking only about revenge or bad karma? Wouldn't it be nice if more often it meant, Hey, thank you so much. I am sure someone will do something nice for you today since you just helped me out.
Goodness and kindness can spread just as readily as greed and selfishness, but in this human experience carnality too often wins, and the eternal parts of us sit back and watch. If this is what our round world symbolizes, give me back the days of the 16th century when explorers were afraid of sailing off the edge.