Dandelion greens can be tolerable if you balance it by using a little less of them and adding in another easier green, like romaine or spinach. And one trick for making any green drink tolerable is to make it with one apple, half a lemon, and an inch of ginger. The lemon and ginger help cut the bitter, and the apple sweetens it all up enough to make it drinkable. Just don't branch out and juice mustard greens, those made the worst drink I've personally experienced, it's not just tough flavor wise, it also burns.