Why?

              

In golf, a caddy (or caddie) is the person who carries a player's bag and clubs, and gives insightful advice.

Where did the term 'Caddie' come from?

When Mary Queen of Scots went to France as a young girl, the King of France  learned that she loved the Scots game 'golf.' He had the first course outside of Scotland built for her enjoyment. To make sure she was properly chaperoned (and guarded) while she played, the King hired cadets from a military school to accompany her. Mary liked this a lot and when she returned to Scotland , she took the practice with her. In French, the word cadet is pronounced 'ca-day' and the Scots changed it into ‘caddie.’

Why are zero scores in tennis called "love"?

In France , where tennis became popular, the round zero on the scoreboard looked like an egg and was called 'l'oeuf,' which is French for 'the egg.' When tennis was introduced in the US , Americans (mis)pronounced it 'love.'

(e.g.15-0) is spoken "fifteen-love"; "to hold to love" means to win the game when serving with the opponent scoring zero points.