@ysmina-- Does your dad have type 2 diabetes or type 1?
If he has type 1, it means his body doesn't produce enough insulin. If he has type two, it means he produces enough insulin but the insulin isn't working right.
I have type 2 diabetes due to insulin resistance. My doctor explained to me what this kind of sugar sensitivity means in detail. I'm much more informed about what's going on in my body now.
My insulin molecules are "deformed" so to speak. So when I eat food and sugar enters my bloodstream, the insulin doesn't recognize the sugars and does not pick them up to turn them into energy for my body to use. So the sugar (glucose) molecules build up in my bloodstream raising my blood sugar.
Meanwhile, since sugar isn't being transformed into energy, my brain sends signals that it requires energy and creates sugar cravings for energy. So I end up eating too much and feeling very tired and very thirsty throughout the day.
Thankfully these symptoms have mostly ceased now thanks to medication. My diabetes medication improves my sugar sensitivity by "fixing" the insulin molecules so that it recognizes and uses the sugar molecules in my bloodstream.
I hope this information helps you and your dad too.