Artificial sweeteners, particularly aspartame, can trick the body into thinking that sugar is being ingested, which causes the body to release extra insulin to help metabolize that sugar.
That would be appropriate if you actually were eating sugar. This insulin spike causes the body to remove glucose from the blood stream and store it away as fat. Meanwhile, the body now realizes that there are no calories coming, but it’s too late because the insulin has already hit your blood stream. This sets the stage for low blood sugar, which, in turn, triggers your urge to grab something super-sweet. If this happened once in a while, your body could manage. But if this is a daily or several-times-a-day occurrence, your blood sugar and appetite can get severely out of whack while your pancreas exhausts itself trying to produce the large amounts of insulin it thinks you need.