Let's be honest with ourselves. Tennis is a weird game. What other sport do you help warm up your opponent? And imagine apologizing for accidentally scoring a goal in a hockey game?  It's a game of politeness and etiquette, but if you could only hear what the other guy is saying in his head after you shank an overhead winner on set point...
Strangest of all, unquestionably, is its bizarre scoring. Even the most astute tennis player is probably unfamiliar with why we go from 15-30-40 instead of 45, and why are we extending our love when it means nothing? Can we blame it all on the metric system?
Alas, we have the answers for you. Er, sort of...it's a bit of a convoluted "he-said, she-said" kind of explanation. 

Answer #1

The origins of tennis date back to the 16th century (France and England can argue about who started it), and in-between dodging small-pox and whooping cough, the people of Europe began playing the beloved sport we know today. The first theory states that since they were without a scoreboard, they used a clock face to track their points.

 1 point brought you to 15 minutes, 2 points to 30 minutes, and since two points were needed to win when tied at deuce, they simply moved 45 down to 40.The next point brought you to 50 (Advantage), and the next point to 60 (Game).And we thought they were bad at adding this whole time.

Seems like a simple answer, right? Well, not quite.
Quite a few historians and experts on the subject dismiss this convenient theory  on the notion that medieval clocks didn't have minute hands until 1680, suggesting that it couldn't have been the reason.
It must have been like the debut of the first iPhone when that minute hand was first introduced.

Answer #2

What is believed to be the real reason for the odd scoring comes from a game called Jeu de Paume (France might have you on this one, England).
A net divided a 90 foot court, and starting from the back, a player would move forward 15ft for scoring first, then up to 30ft, and finally to 40ft as 45ft would bring them too close to the net.

 

The Clifford & Oak Jeu de Paume collection

 
So What's Love Got To Do With It?

The widely accepted theory that we say "love" instead of zero also originates from France (strike two, England). The word l'oeuf translates to egg, and an egg looks like a zero, so the cool kids of the 1600's used l'oeuf as slang, I guess.

L'oeuf sounds like love, and thus our answer for why we use it now. It also sounds more respectful than "bageled".
But the historians had to go and suck the fun out of that answer as well, claiming it originates from a Dutch or Flemish word "lof" meaning "honour".
Even if you're losing 6-0, 6-0 you have nothing to play for but your honour.

Without a time machine it's impossible to know which of these is the true origin, but surely it has to be one of them. 
And they're still much better reasons than one guy once told me; that we say love because a King would say "love" to his Queen instead of "nothing" because he didn't want to sleep on the castle lawn.