feline nutrition

I would love to know why dogs can't eat cat food ,when instinct puts out a raw frozen food good for both dogs and cats ????? Any info that can explain why it's good for both.? I give my 6lb dog royal canine puppy [even though he's 8 yrs. old] with fancy feast as a topper   am  I feeding him unhealthy? I understand the one ingredient  dog food is missing            is why cats should not eat dog food.

Actually, most cat foods are fine for dogs. Cats have more stringent nutritional needs (like taurine and higher animal protein), so most dog foods are not fine for cats.