In case it is of interest:
"The most accepted theory for explaining the strange scoring system [of tennis] is that it reflected Europeans' preoccupation with astronomy, and particularly with the sextant (one-sixth of a circle). One-sixth of a circle is, of course, 60 degrees (the number of points in a game).
Because the victor would have to win six sets of four games each, or 24 points, and each point was worth 15 points, the game concluded when the winner had "completed" a circle of 360 degrees (24×15).
Writings by Italian Antonio Scaino indicate that the sextant scoring system was firmly in place as early as 1555. When the score of a game is tied after six points in modern tennis, we call it "deuce"the Italians already had an equivalent in the sixteenth century, a due (in other words, two points were needed to win).
Somewhere along the line, however, the geometric progression of individual game points was dropped. Instead of the third point scoring 45, it became worth 40. According to the Official Encyclo-pedia of Tennis, it was most likely dropped to the lower number for the ease of announcing scores out loud, because "forty" could not be confused with any other number. In the early 1700s, the court tennis set was extended to six games, obscuring the astronomical origins of the scoring system."
"Why Do Clocks Run Clockwise" by David Feldman
I have also read that a clock face may have been located near the tennis court.
"The clock hand would move a quarter for each point. When the hand moved to sixty, the game was over. Therefore, the scoring would be love, 15, 30, 45, and 60 instead of the traditional 0, 1, 2, 3, and 4. Whoever scored 60 (4 points) first would win the game."
As for the use of "love" for "zero", the French *may* have said "l'oeuf", meaning egg, due to the egg-shaped resemblance of zero (0). It is therefore conceivable that "love" is an English corruption of this, misheard, or mispronounced.