Dog Food

I was just thinking about how if you change the brand of food you feed your dog or cat that the animal becomes ill. I'm starting to think this was done intentionally by the food makers. It makes the dog physically dependent on their brand and when you change the animal goes through withdrawal symptoms, Maybe this is just another conspiracy theory that just in my head but it doesn't make sense to me that change the type of food you feed your pet makes them sick when if the animal was still in the wild and free their food changes all the time depending on whats available. I think companies are mildly drugging our pets.