Today I learned from Miranda July's blog:
"Well 143. Remember that means I love you. 1 for the letter I because it only has 1 letter, 4 for love because of 4 letters of the alphabet make up that word and you know you has only 3 letters so..."
...so, 43 means love you. 13 mean I you. 34 means you love. 41 means love I.