If your puppy can't seem to help sticking his head in the cat litter box for a fresh snack, don't be alarmed -- it's normal behavior, especially for young dogs. Formally known as coprophagia, the habit of eating cat feces is one that many puppies develop, and for several possible reasons. While most puppies don't pick up the habit for life, in other cases, a little owner intervention is required to keep them away from unsavory treat-snatching.
Puppies experience the world through their mouths, and you'll notice that the litter box isn't the only place where a pup finds things he shouldn't be eating. Puppies use their teeth and tongues to discover, sample and chow down on all kinds of inappropriate things, like shoes, stuffed animals and garbage. Getting into the litter box is natural, and when he finds something in there that he can munch on, he's liable to go for it.
Rich In Nutrients
The other most notable reason that a puppy gets in the litter box and ingests cat waste is because it's rich in nutrients. Cat feces are high in protein, which puppies need plenty of during their first few months of growing -- this is the same reason you feed them nutrient-rich puppy food instead of dog food. The protein and vitamins in cat feces are satisfying for a puppy, who has a big appetite and won't hesitate to get his nutrition wherever he can.
Outgrowing the Habit
Eating from the litter box is a habit that puppies generally outgrow on their own. Typically they stop raiding the litter box once they're 6 months old or so, particularly if they're getting the nutrition they need elsewhere. Some dogs don't outgrow the habit though, either because it develops into an obsessive compulsion or because they aren't getting sufficient nutrition from their regular diet. If your dog's diet isn't the problem, you have to step in to break this habit.
When your dog is no longer a puppy but he hasn't lost his taste for unpleasant treats, the most effective way of breaking the habit is restricting his access to the litter box. Place it behind a baby gate or on an elevated surface, so that the cat can get to the box but the dog can't. While there are alternatives, like spraying the cat's feces with bitter apply spray or cleaning out feces immediately after the cat goes, these require you to be on watch virtually 24 hours a day -- by simply putting the box where your puppy or adult dog can't reach it, you protect yourself from any unwelcome surprises.