Well, it is a broad question. Here is the thing. It is possible that there is an issue with the food itself, such as a) it might have rancid fats, b) it might contain something that doesn’t agree with your pup’s system … more importantly, your pup might not be feeling well. Here is how that works – some foods are more attractive/palatable than others and entice being eaten even if the stomach feels upset. For example, we had a similar problem with Jasmine (also since she was a pup) and eventually it turned out she had IBD.
I’d starting by having a close look at the food, followed by a vet evaluation. This might not be an easy thing to figure out but it is important that you do.
There are many factors that can lead up to this type of situation. Have you discussed this with your vet? Have you spoken to your trainer about it or a behaviorist? I’m not sure of your daily routine with your dog as far as walks and training, but those are absolutely KEY in keeping your pack order straight. I would start out talking with my vet and a trainer and take things from there. There is no such thing as a perfec pet. They all require time, work, lots of love and patience. No matter how docile they seem to be, anything with teeth can and will bite, unless properly trained and exercised and knows that they are a part of the pack. Best of luck! Keep us posted.