Can someone explain why diabetics need to avoid sugar most of the time, but sometimes urgently need a sweet?

Blood sugar too low: The urgent need for something sweet may be from low blood sugar, and usually there would be other symptoms. "sweets" get into the system rapidly and can alleviate the low blood sugar. Liquids are the fastest, and simple starches like white bread also enter the system fast. However, routinely eating foods with a high glycemic index is not good for diabetic control due to wide blood sugar swings.