Why do you lose your appetite when you fall in love?

Strong emotion. Love is a strong emotion, not everyone who falls in love lose their appetite though;however some may be so captivated by the opposite sex that she/he may even forget that they need to eat to survive.
I am not sure. In any new pleasurable behavioral paradigm- Dopamine tone is increased. We feel less deprivation and our appetites will be less. Perhaps your hungers have been less of a true hypothalamic mediated nutritional need and more of an externally triggered "wanting" of food.