Because bacteria will grow in the meat once it is thawed and then you put it back in the freezer which just hibernates the bacteria - when you bring it out to thaw again the bacteria just multiplies and next thing you know cooking it just doesn't cut it and you could come down with some life threatening food poisoning. If you have thawed meat that you dont want to eat that day you can cook it into a meal and then re-freeze it, this is ok as long as you only reheat the meal once. You can not keep this meals leftovers for another time you will have to throw them out.