Some riddles about love include: "What makes grown men cry, but humanity would go extinct without it?" and "What causes a sudden increase in heart rate, a loss of memory and overall brain function, but is not a drug?" The answer to both is "love".
Riddles such as these are well suited to Valentine's Day cards and love letters. They are also good for dropping into light-hearted conversation during a date. Love riddles show abstract thinking, romance and a sense of humor. Another example is "What four-letter word is the cause of most of the discord in the world, as well as most of the cooperation?"