What causes your puppy to itch constantly?

itching irritated skin and fur can be caused by several things.

Fleas are a common form of itching. to check for fleas scratch the hindquarters of your puppy over some paper, if you see large flakes of brown/black specs this is known as flea dirt. try a flea treatment such as frontline or advantage

Dry Skin is very common in the winter with dogs, if their doesnt seem to be any fleas and he is still itching, you can give your puppy an oatmeal shampoo, this will help relieve sensitive skin. also try omega 3 capsules, the fish oil is good for dry coats.

Mange or ringworm is a more severe parasite, and will require a test from your veterinarian to check.

Allergies are also common. try some over the counter benadryl to see if that helps relieve the irritation, be careful not to overdose your dog on it.