Since love is based on the reward hormone Dopamine, when the supply of Dopamine that used to create the feeling of love is no longer enough or is not released, the feeling of love is depleted. This means that the person who was in love is no longer in love because they are no longer being rewarded by their brains.
[Fairy tale] Unless you truly love them, then you really never do. If you say you do, then fall out of love, that you really loved them at all. [/Fairy tale]