Certain brands of carrot juice taste sweeter than any raw carrot I ever eaten, yet are made only of carrots, with no added sweeteners. Are carrots sold in supermarkets just not ripe?
If you juice your own carrots you would see how sweet your 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.