If your dog is generally well-behaved and you are in control except for your loving heart when it comes to letting Rex eat whatever his doggies heart desires, the way to “pull apart” the healthy dog food from the unhealthy human food is to become just a little bit tough. Dogs are always begging for food. Give Rex his ration so that he has enough to stay healthy and strong and maintain body weight, etc. Then when his tongue drips and he begs for lasagna, tell him, “You’ve had your share. This is my share so I can stay strong and play with you.” Some people say to ignore the dog at this point and not even acknowledge him/her. I find it hard to do that.
If you are unable to control what your dog eats, that is if your dog gets aggressive when you deny human food, then you’ve got a deeper problem than Rex’s diet. First you need to sort out who owns whom. Cesar Milan in his Dog Whisperer program is an expert on dog-training. In watching his videos I got control of my dog. However, crossing n aggressive dog can be dangerous, especially if the dog might decide to use its teeth to make a statement. If your dog is out of control, I recommend you seek the help of a professional animal behaviourist. Good luck—the answer is probably quit simple but requires insight and backbone.