If you are in space, well you are limited on a lot of things. You can't just jump into a car and drive to the grocery store when you want something. You cannot stock up the space shuttle with boxes of food and drinks. You won't find a whole lot of room in the shuttle. So with that in mind astronauts have to be willing to do different things, and right now that includes drinking urine that has been converted into water or a new experimental sugar energy drink.