Some dogs will lose their appetite because they may not like the food. Others start to eat less or lose their appetite because they're getting older. Sometimes it's for behavioral or stress reasons. When your dog is eating less, but also concurrently losing weight or not acting like themselves – or if I see other symptoms of disease – then I get concerned that there is a process going on, and perhaps a serious ones, such as cancer or hormonal deficiencies. I say the “c” word and people worry, but again, I put that over here in a category of dogs that are not eating much at all. When dogs are doing that, I need to go on a hard search for what's going on.