Eat the eight "Bag of Candies" heart candies listed below.
The candies cannot be mailed or sold on the Auction House, but they can be traded between players.
The Bag of Heart Candies needed for this achievement can be bought using Love Tokens during the Love is in the Air holiday. These bags create a random candy with one of the needed phrases. Because of the randomness of the candies and the fact you will need eight different ones to complete the achievement, you may need to purchase several bags to get them all.
This achievement is needed for the holiday's meta, Fool For Love, which is in turn needed for the main holiday meta What a Long, Strange Trip It's Been.
Added to the original Love is in the Air world event in Patch 3.0.2.
This is the achievement that prevented myself -- and a lot of others -- from getting the main holiday meta (with the then 310% flying mount) in 2009. The original requirement was to create all the candies rather than eat them and was compounded by its source, Bag of Heart Candies', having a low chance of being looted from Gift of Adoration -- which you could get only once an hour. It was possible to go through a lot of bags and still miss just one candy piece. After much grief on the original WoW forums, Blizzard hotfixed it to require only 6 out of the 8. For many, unfortunately, this was not enough.
So to recap the "Be Mine Incident of 2009": Gift of Adoration (1 hr CD) -> Candy Bag (low chance) -> one Candy piece per charge (extremely RNG) -> A lot of unhappy players