It seems that how the term ‘love’ came to mean zero is a secret lost through the ages. There are some theories however but none have been proven. It’s not just tennis that uses the term love to note a score of zero; it is a common British term that is used in any sport.
One theory suggests that the term love means ‘for the love of the game’, I suppose that means that if you’re outmatched and have no chance of scoring you’re playing for the love of the game, hence the term love. Another theory suggests it comes from the French word l’oeuf, which is French for egg, which looks like a zero.
http://www.askoxford.com/asktheexperts/faq/aboutwordorigins/lovenil
