Certain brands of carrot juice taste sweeter than any raw carrot I've ever eaten, yet are made only with carrots, with no added sweeteners. Are carrots sold in supermarkets just not ripe enough?
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 — it tastes pretty much like paper pulp — 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.
Also, the carrot is one of the only vegetables that contains that much sugar. Most vegetables consist of around 2 grams of sugar per 100g, while carrot is at 5 grams.