A. In the Dutch phrase, iets voor lof doen (to do something for nothing), lof means nothing.
B. In French, Lheure means The hour. Since some people think that the 15-30-40 scoring refers to 15, 30 and 40 minutes after the hour, the hour might mean zero.
C. In French, Loeuf means Egg and an egg is in the shape of a zero.
D. Love is a more pleasant word than zero. Since tennis is a highly civilized sport, love evolved as a tactful way of denoting a score zero.