I'm not entirely sure. It appears that she chose this motto "La Plus Heureuse", or "The Most Happy", for her coronation - perhaps she was relieved and happy to finally be married to Henry and crowned Queen! Some people say that she chose this motto to contrast with Catherine of Aragon's motto "Humble and loyal".
My opinion is that this was the time that Anne was at her happiest - she was married to Henry (after a long struggle), she was pregnant with what they both hoped was a son, she was about to be crowned Queen of England and the Reformation seemed to be taking place in England. To Anne, it must have seemed that there was a happy future ahead of her.
For interest, Jane Seymour's motto was "Bound to Obey and Serve", Anne of Cleves' was "God Send Me Well to Keep", Catherine Howard's was "No Other Will but His" and Catherine Parr's was "To Be Useful in All I Do" - don't you think they're apt?!
Sorry I don't know the proper answer, perhaps someone else does!