Carbohydrates are what gives our bodies the needed energy to get through the day and basically the carbs we eat charge up our batteries.
So when your body is low on carbohydrates your energy level drains and when crave carbs it usually means that you're getting low on energy and need those carbs to bring your energy level up.
So if you're craving carbs it's okay and you should eat so you can have the energy.
When you don't eat enough carbs then your body uses your bodies fat to give you the needed energy levels which is why you lose weight on a low carbohydrate diet.
Craving carbs can also be a result of depression or just feeling down and when you consume more carbohydrates more serotonin is created by your body and your blood sugar spikes.