Why is carrot juice so sweet?
If you juice your own carrots you would see how sweet carrots actually are. 

When you juice a carrot, you are extracting the liquid portion (which contains the majority of the sugars) from the cellulose. Since the cellulose is somewhat flavorless&thinsp;&mdash;&thinsp;it tastes pretty much like paper pulp&thinsp;&mdash;&thinsp;you are essentially creating "concentrated carrot" flavor, which is why it tastes so much sweeter than when you eat it whole.

You can also roast a carrot and see how sweet it is. Roasting drives off a lot of the water and breaks down the fiber. Try it; you'll see just how sweet a carrot actually is.