It's because that is the sensation triggered by the part of your taste buds sensitive to the chemical structure of sugar. Humans are hard-wired for three main flavors: salt, fat and sugar. That's because those used to be the three hardest nutrients to find. Now, because we enjoy them so much, those flavors are everywhere, even though we don't really need that much. Our brains and taste buds are still programmed to find sugar (along with salt and fat), however, so when we eat it, we get a positive taste from it. In this way, our body rewards us for eating what used to be a rare treat but is now an addictive plethora.
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surgar is actually a chemical compound that happens to connect to the taste buds.