This is related to another question i asked before, WHY is being on a 750-calorie-max diet unhealthy?

First of all because you're 14; your body is still developing and needs an appropriate amount of calories to do so. Because eating less than 1300 calories per day is unhealthy for anyone at any age. Because if you don't eat that number of calories your body can't function at it's optimum level of performance. Because your body goes into "starvation" mode and hangs on to all the fat it can because it thinks you're starving to death, making it completely pointless to eat so little in the first place.