How healthy is dog food?

Believe it or not, this is something I've been pondering a lot. I feel fairly certain that if I ate exactly the same thing twice a day, day in and day out, no matter how nutritionally complete it says it is--I probably wouldn't be very healthy.

So why is it different for dogs?

I've started giving mine little bits of fresh veggies, small amounts of plain yogurt with live cultures, a little shredded cheese, a little hard boiled egg--just a little something mixed with their dog food to wake up their taste buds. I must say, it has certainly perked up their appetites. Even my picky eater is eating now.

I've learned from experience that when my old dog eats some brands of dog food, he has seizures. He is now on Purina One and has not had a seizure since I switched him to that, and it has been many, many years now. He was seizing once or twice a month before then. Another dog had bouts of diarrhea until I put him on the same dog food. I'm not promoting a certain brand of dog food by any means--this just happens to be the one that seems to work best for my dogs after much trial and error and they eat it, unlike some of the pricier brands I got from my vet.

So I am wondering--how good is it really to keep your dog on one food day in and day out? Is there any real research to support that? (this is my doctoral training talking here)