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.