i went through the web wishing to find the answer or lets say the scientific answer for your question

i found many theories explaining why do we crave

The dieting theory
This theory says we crave the foods we deny ourselves. By eliminatingchocolate completely from our diet, this theory suggests we set ourselves up for failure. The stricter we are about eliminating a particular craving from our diet, the more desirable that food becomes. If this is true for you, you might find that allowing yourself an occasional (even daily) small taste of the previously forbidden food is enough to stave off a binge episode.

The blood sugar theory
The greatest number of cravings occur late in the day, according to the Philadelphia-based Monell Chemical Senses Center. It's a time of day when our blood glucose levels are likely to be low, triggering a high-calorie craving. Having healthy snacks at the ready can stop this craving cold, before you head to the office vending machine.

The memory theory
Our cravings are often associated with pleasant memories and occasions. Often those cravings are rooted in the favourites of our childhood.

The wisdom of the body theory
This theory suggests that our body craves what it needs. This may be true for some cravers, but it's difficult to lay too much credibility on this theory if you think your body needs a litre of chocolate ice cream.

The stress/anger/boredom theory
When you feel stressed, angry or bored, your levels of serotonin, which creates a sense of well-being, drop. Since sweet and starchy foods bump up the secretion of serotonin, you may notice a corresponding improvement in your mood after eating a chocolate bar. The antidote might be exercise, which triggers the release of other feel-good body chemicals called endorphins.

The chronic stress theory
According to Orzech, chronic stress might also contribute to your inability to resist cravings. Studies have shown that stressed-out rats responded by engaging in pleasure-seeking behaviours, like eating high-energy foods.

The reasons behind our cravings can be multiple and complex. Understanding what triggers your cravings is a step towards controlling them.

thanks abdo for the information ..i strongely agree with the blood sugar theary