Why do diabetics urinate so much with a high, normal or low blood sugar.?

No matter what my blood sugar is, I urinate every 15-30 mins. 24/7. I can't understand why because I try my best to be in tight control with it. Still I've asked my doctor and got no where. No answer or explanations on high sugars which I've been a type 1 diabetic for over 25 yrs. great weight, eat well. Just can't understand it, its very anoying to urinate so much.